Add options to update existing PR

[imaustink]

It would be nice if you could run a code mod on branched that were created via landscaper. And maybe even amend the original commit.

[phillipskevin]

Would it handle what you're looking for if you could run Landscaper without submitting a PR.

This way, the process would be:

1. run a code mod, creating a branch from master, which submits a PR
2. run a second code mod, using the branch made in step 1, which would update the PR from step 1 and not submit a new PR

[imaustink]

I think that would be fine. Right now Landscaper throws an error when if the branch already exists. So it would need to add another commit or amending the original if the branch exists.