Export comments

[MaxGhenis]

Fixes #20

I haven't yet tested this.

[evbacher]

Thanks for this suggestion -- a couple of comments/questions:

• We're not currently accepting contributions, as we're trying to add some enhancements. We'll try to evaluate this when we have some time.
• How does this change render the comments in the output? The main purpose of Docs to Markdown is to publish finished content to Markdown or HTML. Comments are not usually part of published output.
• It seems as though you've made many code/style changes that do not relate to exporting comments.
• If not tested, we're loathe to proceed.

On the other hand, don't spend too much time on this right now, since we're not ready to spend time on our end. If we do come back to this, we'll let you know.