Add support for nested git submodules

[phodge]

If you are nesting git repos inside other git repos using git's submodule feature, then you might actually want the parent repo to be the project root not just for itself but also for nested repos.

This PR adds an option g:projectroot_nested_git_submodule which will cause projectroot to read the contents of '.git' marker files and check if they are indicate a git submodule inside another git repo. If this is the case, then projectroot will continue searching up the directory tree for another '.git' marker file.