Rename variable to saveUnpluggedVolCheckBox

savePluggedVolCheckBox was used to determine if we should save the
unplugged volume, and not the plugged volume. Rename variable to match
its behavior.
Add support for saving last plugged volume

Previously, you could only save last unplugged volume, but it's easier
to configure the volume if we save both. Then the app needs less
configuration.
Merge pull request #3 from idbrii/restore-plugged-volume

Restore plugged volume