Feature request: add git submodule support

[jstolarek]

I have a large personal wiki and another smaller one that I share with someone else as a separate repository. It would largely improve my workflow if I could add the smaller, shared wiki as a submodule of my larger wiki repository and then just have it as a sub-section of my larger wiki. Sadly, gollum does not seem to recognize pages in submodules, hence this request to add submodule support to gollum.