Rename --maxdepth to --max-depth

[okdana]

The name of the --maxdepth option is inconsistent with all of the other --max-* options, and aside from being aesthetically displeasing it affects the ordering in the help output and completion results. I assume the wording maxdepth comes from find, and that makes sense on its own i guess, but... it just feels out of place to me here.

This PR changes the canonical name of the option to max-depth, leaving maxdepth as a hidden alias for backwards compatibility.

(I realise this is petty; feel free to just close if i'm being too ridiculous...)

[BurntSushi]

Hah. I like it. You're right about its provenance; I wanted to match find. But you're also right about everything else too. 👍