Properly escape the filepath to rest2html

This PR fixes a bug in `master` that occurs when the filepath to github-markup contains spaces. Currently, the unescaped filepath containing spaces would be passed to CommandImplementation#execute, causing the following error:
```
ludwig:markup dawa$ pwd
/Users/dawa/Test Dir/markup
ludwig:markup dawa$ rake
[...]
GitHub::Markup::CommandError: /usr/local/Cellar/python/2.7.7_2/Frameworks/Python.framework/Versions/2.7/Resources/Python.app/Contents/MacOS/Python: can't find '__main__' module in '/Users/dawa/Test'
```