Add push option to update-prs

[Dan7-7-7]

We've had a couple of people ask how to push new changes after prs have been created. The suggested approach currently is just using foreach with git push. However, the update-prs option is causing confusion, especially as gh sometimes seems able to update the pr with new commits when e.g. turbolift update-prs --amend-description is run, as a side effect of the gh pr edit command (?).

To clear up the confusion and make sure the user gets their intended behaviour we should consider adding a push option to update-prs.

e.g. turbolift update-prs --push to push any new commits.