Add "label" field to replace ugly ids.

[flo-at]

Sometimes mountpoints, temperature sensor identifiers, .. don't look too nice if used through the alarm_id placeholder. It would be nice to have the option to add user-defined labels to them, that will be used as ids for the alarms instead of the internally generated ones.

This is a breaking change regarding existing config files, so this has to be done before the v1.0.0 release.