[feature request] Allow customize PR title when using --create-prs

[upsuper]

It seems currently pr-train -p --create-prs just pick the summary of latest non-merge commit, which works generally fine unless there are already fixup commits in the branch. It also affects existing PRs which usually have new fixup on top of existing PRs.

So it might be handy if pr-train can allow people to edit the PR titles similar to git rebase -i, so that people can have title corrected locally.