Add style attribute for underlines

[philipbelesky]

Hey, love the app! Have been using it as my go to for quite awhile now. I've been making a theme and everything has been going great so far.

The one thing I've wanted to do, but doesn't seem possible, is to change the behaviour regarding the underlining of hashtags and the underlining of the markdown links.

The latter I find distracting or unnecessary, particularly given it highlights both the [actual markdown text](as-well-as-the-url-itself). This also seems inconsistent with how the path is treated within the markdown image syntax (which is unhighlighted).

Not sure if this is a duplicate of #4 — I think even if the two types of link were distinguished it would be useful to style both variants rather than carving out tags as a special case.

[DivineDominion]

I'll not be changing the theme syntax for a while, but will eventually get to it when I work on the Markdown highlighter very soon (see roadmap)