Adds support for channel following and messageCreate type 12.

[Khaaz]

This PR adds support for the new channel follow feature according to: discord/discord-api-docs#1051

This was tested so it support messageCreate type 12 and adds the new messageReference and flags properties to the message object.

A MessageFlags constant is also added in the Constants file.

[Khaaz]

I fixed the naming and added the documentation.
This is pretty much ready, the last question concerns the message content. I used the channel mention to try to replicate discord showing the server where the channel is from. However should we use the raw channel name here?

[Khaaz]

Fixed the documentation and changed the content to the raw data.content without the channel mention.
I thought so as well, it makes more sense to just use the name.