User App Support

[TheKodeToad]

Reposted from engineer-man/piston#672, really sorry about the spam.

Discord now has not only support for installing bots/apps to servers, but also users. Since slash commands only allow one line and no syntax highlighting, that's pretty poor UX - so it's probably best to have a context menu command which executes whatever can be found in the message instead, maybe another which uses a dialog to allow inputting the language ID explicitly.