`use_release_issue()` changes

[hadley]

Based on our discussion at work week, I'd propose that we too new arguments/questions to use_release_issue(): use_pre_release and use_branch.

If use_pre_release is true, then we create two issues a pre-release (the current "prepare for release bullets), and then a release issue, which would gain new bullets for locking the main branch at the start and unlocking at the end.

If use_branch is true, the bullets would replace git pull and git push with (e.g.) pr_init("v1.4.1-release") and merging the PR. (We'd recommend that the PR close the issue automatically with a comment).

(For use_branch = TRUE we still need to think through the post-release process to ensure that both dev and releases sites are updated, and we end up with a dev version on the main branch.)