Add autocomplete functionality (maybe?)

[mdiller]

Slash commands have a cool feature called autocomplete which can help users when theyre typing. This could be pretty useful in a couple different commands (dot clip searching, matchfilter stuff, userconfig/config, dota heroes/items/abilities). My main concern with this is that it'll add more communication between discord and mangobyte, and we're already running into issues with ratelimiting (see #43 ). I'll probably revisit this later once slash commands are more fully implemented in mango and see how we're doing with the rate limiting issue.