Fixed mathjax underscore error

These changes are in reference to issue [#90][1]. The changes add the [code-friendly][1] option to the markdown2 editor when either mathjax or highlight.js is enabled.  This will cause the parser to skip leading, trailing and intra-word underscores, which otherwise are interpreted as `<em>` tags.

I also fixed a mistake in the mathjax config file (mathjax.html) that improperly included extensions necessary for rendering tags such as `\binom`.

Starting a new line with `\\` is still not working properly (still need three backslashes). I'm not sure how to go about fixing that one.


[1]: https://github.com/revolunet/sublimetext-markdown-preview/issues/90
[2]: https://github.com/trentm/python-markdown2/wiki/code-friendly