Option to turn off for certain file extensions (i.e. .yml)

[jlperla]

While I love seeing the math typeset in issues/etc, it causes havoc to the built-in github viewer for some files. In particular, .yml files for travis. e.g.
https://github.com/tornadoweb/tornado/blob/master/.travis.yml

Turn it on, and you will see what I mean. If you added a setting to just list post-fix extensions to ignore for the webpages, that would be enough for me. Better false positives than false negatives.