Add power=plant to area creation rules

[joconnor-ecaa]

Replicated rule for e.g. power=generator.

Resolves #609

From issue:

Posting an issue as instructed by https://wiki.openstreetmap.org/wiki/Overpass_API/Areas. My use case involves querying for objects within the area of a power plant way.

The area creation rules as specified here include power=station, power=generator, power=sub_station and power=transformer but no power=plant. Most power plants have landuse=industrial but it if the plant is left without a name, no area is created. It also seems that the landuse=industrial tag may be disputed for power plants.