* 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.

💥

This was 👍'd in the dev chat, I forgot to make a PR for it (whoops!)

* Add IChannel.Nsfw, RequireNsfw precondition

* Refactored IChannel.Nsfw to IsNsfw, removed NsfwUtils

Per pull-request feedback

* proper nsfw channel check

* Overloaded AddModuleAsync with Type

* Overloaded RemoveModuleAsync with Type

* Use expression-bodied method for consistency

It's impossible to add non-reference types to the map, so why allow pulling them out of it.

Resolves #584

* Improve parameter precondition type safety

Also removes some terrible code which was left over when I first
implemented parameter preconditions. I don't know why that was there.

With this commit, parameter preconditions should be much safer as they
use generic methods instead of janky casting of objects.

* Remove generic CheckPreconditions method

Fix detection of IDependencyMap impl

A null conditional operator was required at line 70 to avoid a nullref.

Not pretty, but it works.