Added support for documenting MATLAB application files

[ilent2]

This commit adds support for documenting the .mlapp application files.

The documentation is sourced from the app metadata XML files, the reader
uses ElementTree and ZipFile to read the .mlapp file and parse the XML files.

I've added an example application file from R2018a. It is probably a good idea to
test it with a more recent Matlab version too.

To document apps I've added an application directive and a app reference
and an example usage to tests/test_docs/index.rst

It might be good to add other options for displaying additional information, such
as which version the mlapp is was built with, including the preview image, or
toggling the inclusion of the summary or description text.

[joeced]

@ilent2 Thanks for the contribution. I've never used .mlapp's myself, thus never had the need for adding it. As with #96 , I'll try to get a new release out this week.