Option to use underlined headings

[casey]

I prefer to use the underlined form for H1 and H2 headings, like so:

h1 heading
==========

h2 heading
----------

I personally find that it sets them off a bit more, and makes it easier to see the structure of a document at a glance, when reading the raw markdown.

What would you think of an option to use underlined H1 and H2 headings when formatting a tree as CM?

This seems like a simple change, so if you're amenable to it, I'd like to give implementing it a shot.

[kivikakk]

Absolutely! Per #136, consider refactoring a bit so we can group output formatting options.

[casey]

I hacked together a super-gross regex-based solution for my original use case, so until it falls apart, I probably won't be able to get to this.

[kivikakk]

Sounds good! I'm going to refactor the options now before any of these might happen to get underway. (→ #139)