Add --edit flag

[alicederyn]

The following should be valid commands:

• git rex --edit COMMIT — Opens the commit message of COMMIT in an editor prior to executing the script, similar to git cherry-pick --edit
• git rex --edit — Opens a template commit message in an editor; executes the user-supplied script and commits.

Both will need to implement the behaviour of the git commit --cleanup flag prior to executing the script. Optionally, we could add a new default mode that does not ignore comments in script sections, as that's probably going to be pretty convenient.

Automated commit created with git-rex

The following commands were executed:

```bash
# Enter your script here. It will be executed, and all files created
# or changed committed to your repository.
```

# Please enter the commit message for your changes. Lines starting
# with '#', except those in script sections, will be ignored, and an
# empty commit message, or one with no script commands to execute,
# aborts the commit.
#
# On branch foo

Both should respect the core.editor config, if set.