Add support for database key files

[Terrance]

This adds a --key argument pointing at a KeePass key file, when used alongside a password to decrypt a database. (pykeepass already supports key files, so it's just a case of wiring up an option to pass a filename through.)

I've also made this partially available to the export() method, though not yet to the mv/cp commands which use it as that will change the command syntax.

[cizmazia]

Merged, thank you!