Support parsing the project manifest file

[joshtemple]

This project does not currently support parsing the project manifest file, but it should be possible because the manifest file follows a syntax that is essentially LookML.

To support this file, we would need to review the keys defined by Looker, update lkml/keys.py, plus ample test cases (ideally with public Looker projects found on GitHub) to confirm it works in all cases.

Also see: #75