Split API and CLI

[michaelbaudino]

We should remove the CLI script from this project to make it a pure Ruby library.

CLI should be moved to a addic7ed-cli project which uses addic7ed-ruby (and powered by Thor or GLI).

It should take away from the API gem both the actual CLI code (the bin/addic7ed file) and the actual downloading of the subtitle file (the API gem should only return the URL).