Regions replaces Mark, Selection. Refactored into a plugin.

Regions feature is now a plugin. Since there was an overlap in functionality between marks, selections and regions, they are all merged into a single plugin.

I've updated the major version, because we break old mark, selection and regions APIs.

Regions API is broken to a lesser extent – I've renamed `startPosition` and `endPosition` to just `start` and `end`, renamed some mouse event names, too.