ability to create multipolygons

Right now it overloads merge to just take two or more multipolygons and closed ways and try to make them into one multipolygon. It might be useful to have an *extend* operation for single-selected areas so that a user can just draw the hole instead of drawing an area and multiselecting.

How should this deal with intersecting (not contained) polygons?
- create the invalid geometry
- union all intersecting polygons (there would be undefined behavior in [some cases](https://gist.github.com/ansis/38d281370f1f823edda5))
- no action

It can extend multipolygons with rings formed by multiple ways, but it can't create new rings with multiple ways. Not going to implement this unless anyone sees a good reason to.