Add support for mentions to ChannelMessageSend function about discordgo

[bwmarrin]

Possibly parse message content for an @id tags and add those into the mentions field. I'd like to have a variable of some sort to enable/disable parsing, also another variable to convert @id tags to just @Username or just Username for when you don't want to actually do a mention.

[bwmarrin]

Mentions have changed again and require @name in message and the ID in mention array. In a week or two they care changing (on Discord's side) to only needing @id in msg and not needing mention array. I'll just wait for that change before making any new updates.