Fetchart plugin: new google custom search engine backend

[lcharlick]

This restores the fetchart plugin’s google backend (see #1760) using the google custom search API.
Configuration options are identical to the lyrics plugin.

[sampsyo]

This is awesome! Thanks for bringing this back—and double thanks for adding the readable, mock-based tests. ✨

Can I ask for one small change? Although using the configuration keys from the lyrics plugin was a good idea, those names don't fit well with the rest of beets' configuration. (I regret settling on those names in the first place.) Can you change them for this plugin to something more consistent and lower-case? For example, google_key and google_engine would be fine, as would apikey and engine underneath a google: subsection.

[lcharlick]

OK, I renamed the keys to google_key and google_engine.
Glad to hear the tests are alright - I’d never used the responses library before.

[sampsyo]

Awesome; thank you again. I'll merge this now.