Add docstring section "properties"

[joeced]

Would it be possible to add a new docstring section properties as an alias for attributes. This would help write MATLAB docstrings more easily with https://github.com/sphinx-contrib/matlabdomain. Also see sphinx-contrib/matlabdomain#105

[McSinyx]

Also see GH-22. FYI napoleon's been moved to sphinx.ext, so we should file an issue over Sphinx instead (I've been procrastinating on this for quite a while now).