- Add `-h` and `--help` flags to print out basic usage.
- Refactor determine_stat_variant so that it checks to make sure the
value from `stat` is a epoch timestamp, and exit noisily if it is not.
- Change `determine_watcher` to set the `OPT_WATCHER` global directory
directly instead of echoing it then being called from a subshell. There
was an errror that the subshell was obscuring.
- Add a test to reflect the bug.
