Pass down zoom level to the renderer

[pnorman]

As a follow-up to #23 (comment)

All renderers support accept an optional hashmap of key:values (mapnik/mapnik#2553), which are apparently passed down to the postgis plugin.

Do we want to pass down zoom from avecado? It would make SQL queries that are zoom-dependent easier to write, but data styles using those queries would become less compatible (e.g. not work with mapbox-studio)