add support for specifying lat-lng on URL
support default line size option
- add 'feature selection' on mouseover:
  - build a collision map for each tile by rendering to a separate offscreen canvas
  - generate a random 24-bit RGB color value for each geometry, keep a hash of color->geometry pairs
  - render the geometry to the collision map using that color
  - on mouseover, get the pixel value of the collision map, and check the hash to see if it matches a geometry
  - display nam info for any matching geometry
- added POI rendering
factor out GeoJSON coordinate transforms and rendering
support more types (properly rendering MultiLineString now)
make tile_min and tile_max properties of tile