Allow beet submit to accept an API key from the command line

[RJSent]

Hi all,

Most of my config files are publicly visible and are deployed in a read-only state with GNU Guix. As such, I don't have an easy way to store an Acoustid API key in the Beets config file without going through a whole song and dance. (Rename the symlink, copy the file contents, paste into a new config.yml, add the API key, submit fingerprints, delete the new config.yml, restore the symlink.)

I (and hopefully others) would find it grand if beet submit could accept an API key from the command line when it was invoked.