WIP of #125: screen locking detection

[dschep]

separate backends implemented via a 'multi' meta-backend.
Uses config like this:

backends: [multi]
multi:
    locked:
        pushover:
            user_key: user-api-key
    unfocused:
        default: {}
    focused: {}

This config would cause no notifications if the shell is focused, desktop
notifications when unfocused, and pushover notifications when the screen is
locked. Freaking magic. 🎉

Currently supports linux 🐧 with xscreensaver, lightlocker(detection seems iffy), and Gnome.

There's also a --locked-only cli option.

Todo:

• macOS 🍎 support
• Windows 🎏 support
• a better solution than the multi module
• documentation

[coveralls]

Coverage decreased (-4.5%) to 80.91% when pulling 7e30cc4 on locked into c7cbf8f on master.

[coveralls]

Coverage decreased (-8.3%) to 76.479% when pulling 8ce5240 on locked into 7a79b95 on master.

[coveralls]

Coverage decreased (-8.3%) to 76.479% when pulling 8ce5240 on locked into 7a79b95 on master.

[coveralls]

Coverage decreased (-8.3%) to 76.479% when pulling 8ce5240 on locked into 7a79b95 on master.

[coveralls]

Coverage decreased (-8.3%) to 76.479% when pulling 8ce5240 on locked into 7a79b95 on master.

[coveralls]

Coverage decreased (-7.7%) to 75.676% when pulling fa49894 on locked into bd06e7f on master.

[dschep]

Closed in favor of #170