Update the example precondition to use IServiceProvider

Update cmd docs to use IServiceProvider

* Update intro.md

* Update intro.md

* Update intro.md

* Update intro.md

* Update intro.md

* Update intro.md

* Update intro.md

Added/clarified some comments in structure.cs

Fix spelling

Update the quickstart structure example to rc3.

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.

Remove CommandService.BuildServiceCollection

💥

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

* Made GetOrCreateUser always call AddRef and added check to PRESENCE_UPDATE to avoid readding users who have been removed from guilds

* Removed AddRef as per dev guild discussion

* remove tip in docs about SocketEntity.Discord

* Expose the 'Fields' collection on EmbedBuilder

After some discussion I decided that there was really no reason to keep this private, and it didn't really go along with the rest of the design of the EmbedBuilder.

This is NOT a breaking change.

Exposing this property should not have any negative effects.

* Don't allow EmbedBuilder's Fields to be set to null

* Add IChannel.Nsfw, RequireNsfw precondition

* Refactored IChannel.Nsfw to IsNsfw, removed NsfwUtils

Per pull-request feedback

* proper nsfw channel check

* Replace Where.FirstOrDefault with FirstOrDefault

* Replace Where+FirstOrDefault in ClientHelper

* Overloaded AddModuleAsync with Type

* Overloaded RemoveModuleAsync with Type

* Use expression-bodied method for consistency

This should fix the appveyor build.