use git for renames when appropriate

* if the file is tracked then use `git` to move it
* if the file is _not_ tracked then just use `rename()` as before
use git for deletions when appropriate

* if the file is tracked then use `git` to delete it
* if the file is _not_ tracked then just use `delete()` as before
add global variables to control use of git for renaming and deleting
Merge pull request #8 from daveworth/feature/use_git_for_rename_and_delete

use git for rename and delete when appropriate