Add remote flag to submodule update functions

[mandeep]

This PR adds a --remote flag to the CLI update subcommand. When the flag is given, every submodule in the specified aggregate repository will be updated to its latest revision.

This is the relevant GitPython API reference regarding the keyword argument that is called:

to_latest_revision – if True, the submodule’s sha will be ignored during checkout.
Instead, the remote will be fetched, and the local tracking branch updated.
This only works if we have a local tracking branch, 
which is the case if the remote repository had a master branch,
or of the ‘branch’ option was specified for 
this submodule and the branch existed remotely

All tests pass locally with my personal access token.