allow to use both --locale and --version params for 'wp core download'

Ugly coded, but not so different from source. If the requested combination of locale+version is not available, the script fails like with wrong version parameter.
Tested setting it_IT and de_DE parameters and various versions.
Merge pull request #666 from MattiaG/patch-1

allow to use both --locale and --version params for 'wp core download'