Expose backups opt as stdin argument

[zacharyabolton]

Exposes the backups boolean option as a stdin argument, allowing the user to
specify whether or not to create backups of files that are modified.

Moderately tested, but not extensively. This is a simple change that relies on
existing core logic without introducing new logic.

Really just a matter of adding a new stdin argument fmt_opts array in the
erl_tidy_prv_fmt.erl module.

For folks who wanna live life dangerously, they can now run erl_tidy without
backups filling up their directories.