Add `--cleanup` flag for message cleanup modes

Dolt currently applies a single hardcoded cleanup mode to all user-provided commit messages. This matches git's strip mode and is the right default, but there is no way to opt out.

`git commit` and `git merge` expose this via `--cleanup` [[1](https://git-scm.com/docs/git-commit#Documentation/git-commit.txt---cleanupltmodegt)][[2](https://git-scm.com/docs/git-merge#Documentation/git-merge.txt---cleanupltmodegt)]. The four modes are strip, whitespace, verbatim, and scissors.