Add optional underline support

This is `*italic*` and this is `_underline_` when enabled. (This would still be `__bold__`.)

I was wanting this to be `___bold underline___` but I wasn't sure how you'd feel about changing the triple emphasis renderer API.

Instead of making a separate callback for underline, it would be more flexible to provide the original text to the callback. Again, it would require changing the renderer API, so I went down this road.