Allow the Discord bot's prefix to be a mention

[newgrp]

Currently, trying to set the bot's prefix to mentioning it (via !prefix set "@DiceParser ") leaves the bot in a broken state where it does not recognize any commands, at least AFAICT. It would be useful to be able to set the bot's prefix to its mention, as mention-to-invoke is easy to remember and virtually guaranteed not to conflict with any other bot.

Any resolution to this issue would ideally work even if the bot had a nickname in the server. It would be nice if the mention could be embedded in an arbitrary prefix string, but I think most users would be fine if the only supported mention prefix was "@DiceParser ".

[obiwankennedy]

Done