Uses local html2haml when possible

This is @Tinmonkey's [patch](https://github.com/Tinmonkey/sublime-html-to-haml/commit/ba0fe15db1203830f768b8cb873dad1d9b6f8ba6) which adds support for a local `html2haml` which was rejected in #5.

I don't see any reason to have a setting for this. Do you still want this to be added before merging this patch?