* Apply consistency to attributes in the commands extension

This resolves #527.

Not sure if I missed any, putting this up for review.

* Allow preconditions to be used multiple times (for use with groups)

* Addition of FailOnTooManyArgs

* Correct name & only pass in bool

* Fix null channel being passed in RequirePermission preconditions

* c#7 pattern matching

Resovles #870

commit b7fb44a94f
Author: Hsu Still <341464@gmail.com>
Date:   Sat Sep 2 14:34:18 2017 +0800

    Fix more async naming violation

commit e6912e2d02
Author: Hsu Still <341464@gmail.com>
Date:   Sat Sep 2 14:23:04 2017 +0800

    Fix incorrect null xmldocs string

commit da8d23222d
Author: Hsu Still <341464@gmail.com>
Date:   Sat Sep 2 14:17:12 2017 +0800

    Fix CheckPreconditionsAsync

commit 992407407a
Author: Hsu Still <341464@gmail.com>
Date:   Sat Sep 2 14:07:12 2017 +0800

    Add Async suffix to abstract Task methods

* Improve Boolean Property Summaries

Having the `CaseSensitiveCommands` property summary asking a question whenever Intellisense is invoked seems a bit nonessential instead of *properly* explaining what exactly it does. It would be better if instead, it stated it's use to be more comprehensible to the reader.

### Changes

- Edits the summaries of `CaseSensitiveCommands` and `ThrowOnError` to follow a more methodical convention for boolean property summaries (`Determines whether X ...` rather than `Should X be ... ?`).

This is just a small change to improve upon the current documentation, so it shouldn't conflict with anything.

* "DefaultRunMode should also be "Gets or sets blah blah blah" to be consistent."

* Add NullableTypeReader.

Primitives now also load a NullableTypeReader<T> and any value types that get a typereader added will also have a NullableTypeReader<T> added for it.

* Remove unnecessary null check.

* Added docstrings.

Why am I such a bad programmer? Maybe I'm just bad with git. Maybe I'm
just bad in general. Maybe I should resign from programming.

* Fix CalculateScore throwing on missing parameters

* Bump to version 1.0.1

* Allow commands to return a Task<RuntimeResult>

This allows bot developers to centralize command result logic by
using result data whether the command as successful or not.

Example usage:
```csharp
var _result = await Commands.ExecuteAsync(context, argPos);
if (_result is RuntimeResult result)
{
    await message.Channel.SendMessageAsync(result.Reason);
}
else if (!_result.IsSuccess)
{
    // Previous error handling
}
```

The RuntimeResult class can be subclassed too, for example:
```csharp
var _result = await Commands.ExecuteAsync(context, argPos);
if (_result is MySubclassedResult result)
{
    var builder = new EmbedBuilder();
    for (var pair in result.Data)
    {
        builder.AddField(pair.Key, pair.Value, true);
    }
    await message.Channel.SendMessageAsync("", embed: builder);
}
else if (_result is RuntimeResult result)
{
    await message.Channel.SendMessageAsync(result.Reason);
}
else if (!_result.IsSuccess)
{
    // Previous error handling
}
```

* Make RuntimeResult's ctor protected

* Make RuntimeResult abstract

It never really made sense to have it instantiable in the first place,
frankly.

* Allow arbitrary attributes to be added to commands

I still don't approve adding type info back into commands, so
I decided to use this solution for allowing arbitrary attributes to be
added to commands.

Add attributes property to ParameterBuilder

Add Attributes properties to info types

* Why on earth git

* Add using for system so that Attribute can be used

* C#7 features in commands, CommandInfo in ModuleBase

* Update TypeReaders with C#7 features and IServiceProvider

* Add best-choice command selection to CommandService

* Normalize type reader scores correctly

* Fix logic error and rebase onto dev

* Change GetMethod for SetMethod in ReflectionUtils

Should be checking against setters, not getters

* Ensure args/params scores do not overwhelm Priority

* Remove possibility of NaNs

PreconditionGroup quick fix

It didn't make much sense

* Add grouping of preconditions to allow for flexible precondition logic.

* Fix checking Module Preconditions twice (and none of the command's own)

* Fix command preconditions group 0 looping over every other precondition anyway #whoopsies

* Use custom message when a non-zero Precondition Group fails.

* Fix doc comment rendering.

* Refactor loops into local function

* Considering a new result type

* Switch to IReadOnlyCollection<T> and fix compiler errors

* Revert PreconditionResult -> IResult in return types - Change PreconditionResult to a class that PreconditionGroupResult inherits.

* Feedback on property name.

* Change grouping type int -> string

* Explicitly use an ordinal StringComparer

* Full stops on error messages

* Remove some sillyness.

* Remove unneeded using.

Also fix an issue where RequireBotPermission may throw if used in a
non-guild channel which required guild permissions.

Parameter preconditions were always getting the empty service provider,
even when a custom one was provided in ExecuteAsync, which means that
preconditions which use services cannot work properly.

💥