Add support for modifying Polygons

[acannistra]

#11 introduced support for <create> nodes in Polygons, but did not implement modifying Polygons when they intersected with another feature (e.g. https://github.com/trailbehind/changegen/pull/11/files#diff-0f0589840ba0db708b762b8eeea2d8b2b44258b433147acc90ab09b013292fd3R690-R702)

We might want to allow for the creation of <modify> nodes whenever a feature intersects with a Polygon. This is complicated when the polygon is actually a relation, because we don't have the ID of the Way that represents the Exterior of the multipolygon (or polygon w/ holes).

In these cases we need a way to determine the outer ring of the Polygon so we can modify it. This is complicated and requires a design, so we're leaving it as an open issue.