Support submodules

Fixes #6. I'm using an [ini parser](https://github.com/shockie/node-iniparser) that I found on github instead of writing my own for safe parsing of .gitmodules. 

It handles the [empty](https://github.com/jerizm/test/tree/emptyfile) and [semi-malformed](https://github.com/jerizm/test/tree/malformed-gitmodules) cases gracefully. 

Also git won't even let me commit a completely malformed config file. It returns `fatal: bad config file line 1 in /Users/jerry/GitHub/test/.gitmodules`