Add submodule sources

[vmrob]

Currently, the best available method to develop first party dependencies alongside a top-level project is to either use a directory source which currently provides no means to track changes and incurs the overhead of a full clone every build OR manage the dependencies entirely through needy leading to workflow changes for developers and the inability to use normal git tools.

I would propose that needy provide a submodule source that leverages the best of both the directory source and the repository source: management via standard git tools and file/folder modification tracking via git. As an added benefit, I don't think specifying dev mode for submodules will every be necessary which greatly simplifies developer workflow and eliminates a very common user error--leaving a need in dev mode.

libraries:
    mylib:
        submodule: third-party/submodule-dir

[vmrob]

@ccbrown any thoughts on this?