Added support for polygon/polyline

The `<polygon>` and `<polyline>` elements are just simplified shortcuts for `<path>`, so they were fairly simple to implement (see http://stackoverflow.com/a/9709153/433558).  This makes it much easier to just drop-in unchanged D3 libraries that you don't necessarily have control over the source code of and don't really want to maintain a local mirror with slight modifications.

Hopefully I haven't missed anything glaringly obvious.