Add ST3 support

Nice plugin!  I'm using Sublime Text 3 and wanted to try this out on my markdown files.  This is the result:

- Fix pep8 / whitespace
- Add support for Python 3 / ST3 (I think)
- Add markdown extensions to default list

The thing I'm not sure about is if more is needed to support ST3 in an official capacity (does there need to be an ST3 branch?  Update readme?).  It works for me (highlights words), but I haven't tested it in ST2.