Git: add push url

[jensens]

I need a way to add a push URL. At the moment at
https://github.com/vcs-python/libvcs/blob/master/libvcs/git.py#L391
it is already possible to add an URL or overwrite one. But the --push parameter is not supported (see https://git-scm.com/docs/git-remote).
I would like to be able do something like:

repo.set_remote("origin", "some-url", push=True)

I can provide an PR if this is something considered for merge.

[tony]

@jensens Yes, please add a PR! (And a test if you can!)