Add new webhook commands

[LewsTherinTelescope]

Adds two commands: mod, which exposes just a couple options (channel and edit link) to avoid option fatigue, and webhook, which exposes a fuller set (channel, edit link, username, avatar URL, webhook link/ID). (They use the same handlers, just a difference in what the metadata says it accepts.)

There's also a bit of commented-out code to prep for attachments support once the option type drops, but obviously can't be tested yet.

Advanced feature if you want to play around with it: starting the message content with the string RAW: will tell it to interpret the rest of it as raw message JSON, so you can do embeds and stuff that way.

[LewsTherinTelescope]

Changed to draft, as I have a better version in the works but it depends on #51 and thus can't be merged yet