Allow configuration of editor `target`

[kipz]

Initial requirement is allow setting the base branch (or target branch), source branch and PR title.

Ultimately, we should make the target way more configurable:

• Set PR body & reviewers
• Configure auto-merge policy
• commit directly to branch X (without PR)

So the design should allow easy extension, but at the moment only cater for base branch.

[cdupuis]

Also it should be backwards-compatible.

Initially we need the following three things:

• base branch to run the editor on
• name of branch to create for raising the PR
• title of PR

[kipz]

Why wouldn't we want a PR body if we allow setting of the title?

[cdupuis]

We could. But the PR body is constructed based on ChangeLog entries on PMV. So if you wanted a PR body you could use just project.describeChange().

[kipz]

In master.