Match latex in headers

Here's a pull request to match latex in headers.

Please note that I also changed the pattern for pandocAtxHeaderMark.  I honestly don't know what you are trying to match with that pattern.  I am guessing you were trying to not match a # that is preceded by a \, but what was happening is that anything that followed a \ was not being matched.  I swapped the 0 or more # (#*) for 1 or more # (#\+).  Please let me know if I actually messed something up here!