fix: parse RuleMode::Directory from 'dir' as well as 'directory'

[adam-gaia]

Hello,

The help message shows

 -r, --rule <ARG_RULES>
          Pass rules via CLI. -r/--rule `/remount/this:/to/this:<file/dir>`

which lead me to believe I should write dir instead of directory as the file mode. Eventually I figured out that only directory is acceptable.

This is a quick PR allows parsing either 'dir' or 'directory'. If you would rather not accept this PR, please consider updating the help message to spell out the full word directory

[adam-gaia]

I really love boxxy, btw! Best of luck with your health!

[queer]

Thanks!