Add support for reading and writing polylines

[frewsxcv]

https://github.com/georust/polyline

[michaelkirk]

Polyline is a bit different from the other geometry formats we've supported thus far. Primarily it's only for encoding a single line string.

Some outstanding questions:

1. Do we handle multi line strings? (and how?)
2. What do we do for other geometries?

[frewsxcv]

Agreed the writing behavior would need to be discussed, but (I think?) the reading behavior seems pretty straightforward. Read the string, write linestring(s) as the output.