Tweak markdown escaping to be less eager.

The markdown escaping was still pretty overzealous, escaping -, +, and #
characters even when they didn't affect the markdown output. This made the
source text annoying to read.

I've made the regexes more complex to handle these cases.

Testing done:
* Verified my assumptions about how these characters are treated in the source
  text.
* Ran python and javascript unit tests.

Reviewed at https://reviews.reviewboard.org/r/5005/