Performance, features, and usability improvements

• Switched to top-down rendering, yielding performance improvements
• Add a dot-density gamma feature to thin out especially dense clusters
• Add support for multiple layers, making it possible to include more
  than one GeoJSON featurecollection in a map. #29
• Added flags that let you optionally avoid simplifying lines, restricting
  maximum tile sizes, and coalescing features #30
• Added check that minimum zoom level is less than maximum zoom level
• Added -v flag to check tippecanoe's version

Top-down tiling

Splits tiles from the top down instead of building an index of features and working from the bottom up.