Refactor xkcd code to be more modular

[royh02]

The current xkcd code is not very modular, making other refactoring efforts hard. I suggest the following:

• [ ] Move all async logic (including sending messages) to the main bot.py file and only have xkcd.py handle retrieving xkcd comic. This way, xkcd.py functions more like a util file, which it is.

• [ ] Change the method of handling messages to handling args so that it's consistent with other commands. After finishing the task above, this part becomes easier.

• [ ] Add a small description about what the command does using description and brief. Refer to other commands as examples. This should be quick and fun! 😄