Message.ContentWithMentionsReplaced() replace role and channel mentions.

[VagantemNumen]

Currently Message.ContentWithMentionsReplaced() only replaces the User mentions, but not Channel and/or Role mentions.
It maybe nice to have them replaced too.

[ToadKing]

Message.ContentWithMentionsReplaced() also does not replace user names with their proper guild nickname

[jD91mZM2]

Made a pull request for this 😛

[bwmarrin]

Fixed with #375