Set the compiled file in dist as the main file

This lets people just import @mapbox/mapbox-gl-draw instead of having to
import @mapbox/mapbox-gl-draw/dist/mapbox-gl-draw.js. It's standard
practise to set the compiled file in main, and most other packages
including mapbox-gl does so.

package.json

@@ -5,7 +5,7 @@
  "homepage": "https://github.com/mapbox/mapbox-gl-draw",
  "author": "mapbox",
  "license": "ISC",
- "main": "index.js",
+ "main": "dist/mapbox-gl-draw.js",
  "style": "dist/mapbox-gl-draw.css",
  "browserify": {
  "transform": [