Improvement(?): Write our own inline/Discord markdown parser.

[a2aaron]

Right now, we have to fight pulldown_cmark on a bunch of stuff, because it's trying to do a bunch of stuff for parsing Markdown. This includes

• eating newlines
• adding <p> tags to messages
• using <pre> and <code> instead of a <span> that we can style
• not handling underline

This is annoying and suggests that we might be better off writing our own markdown parser. This doesn't need to implement the whole Markdown spec, but instead just the inline formatting stuff (and probably doesn't even need to be super good at it's job, so long as it can get most of the cases). This would make handling blank newlines much easier as well.

[a2aaron]

Implementing this would make #14 and #15 much easier to implement as well.