Adds a field to individual sources that allows overriding the --recurse flag

[paulhovey]

When doing gitman update, there's an option for --recurse (which, this could probably be documented better, as it's only mentioned directly in the API page: https://gitman.readthedocs.io/en/latest/interfaces/api/ )

Sometimes I have multiple repositories but only want to recursively update some of them, and this is the functionality in this PR. There's a new flag in gitman.yml repositories that will override the --recurse flag if it is present, and default back to that flag if not present. That means existing functionality is unchanged, but users can choose to either recurse into directories of their choosing, or recurse into all except directories of their choosing.

[jacebrowning]

I'm not sure what --recurse flag you are referring to.

The --all flag in gitman update means to also pull in the latest versions of their dependencies rather than just checking out the last pinned version.

Does --depth perhaps fulfill your need?