Compare results vs fixture

Currently in master output is written to disk but not actually compared with previous results.

- Adds orig geometry to fixture for easier debugging (e.g. `geojsonio <fixture path>` to see orig geometry vs cover). Example: https://github.com/mapbox/tile-cover/blob/assert-output/test/fixtures/russia_out.geojson
- Compares resulting tile-cover with fixture on disk -- mismatches result in test fail

Prep for further vetting `tile-scanline`.

cc @morganherlocker