Add more ways to call commands

[L0neGamer]

This'll require a big-ish rewrite in places but should be fine, right?

Firstly, let the bot commands be called by @ ating the bot followed by a command; for example, @tablebot quote add "this is a quote", or whatever quote syntax is.
Secondly, let an administrator change the prefix for commands so they can prevent overlaps, and let this prefix be multiple characters if they wish; for example, tb!quote add "this is a quote", or ~quote add "this is a quote"
Thirdly, add slash command support. I'm not sure how slash commands work, but it'd be good to have them.

[finnbar]

(1) and (2) are really cool ideas, and would require only a minor rewrite - currently the prefix is a single argument retrieved from the environment, but it could be stored in a database and updated by a mod. You'd also need to update the prefix parser in Tablebot.Handler.Command - currently it just checks for chunk prefix (a direct match on the prefix), but you could extend it with chunk prefix <|> (@tablebot parser). Note that Discord won't return this as @tablebot but <@somenumber>, which will need to be found.

I'm not sure discord-haskell supports (3) yet, and as such I don't think it'll be possible. But maybe in future!

[the-Bruce]

Unfortunately for 3, the author of discord-haskell has announced that they have no intention of adding that, so we'd either have to find another framework that does (which I don't believe there are any, yet), or write our own. Also, as slash commands do the parsing for you, we'd need to migrate fully to smart parsers to be able to support them transparently (or fully separate slash commands from prefix commands, which makes certain kinds of plugins rather hard to write.

[L0neGamer]

This pr adds application commands and therefore some level of slash commands to the bot.

#118