Map operator

This is far from finished (requires docs, tests, improvements!) but PRing in case anyone wants to have a look at efforts from last night’s Open Data Maker Night :)

The operator takes a single argument – a URL to a custom transform. There’s an example transform here: https://gist.github.com/andylolz/7794290

This is a bit messy at the mo. The messiest thing is due to all of this parsing and stringifying. That @davidmiller suggests we stop passing strings around everywhere and make the switch to binary-CSV, which I agree with.

Refs #21.