add fit-bounds and global map-id configuration support

[CaryLandholt]

Add mapId to the service configuration to use this id globally or as a fallback when not specified in markup.

mapboxService.init({ accessToken: 'YOUR_ACCESS_TOKEN' , mapId: 'YOUR_MAP_ID' });

Add the fit-bounds markup attribute to force the map to fit the feature bounds.

<mapbox fit-bounds>
  <!-- map content here -->
</mapbox>

[sllvn]

Sorry for the slow turnaround. Took a look at your fork and the geojson.html example map wouldn't render for me when I added the fit-bounds attribute.

There already exists a scale-to-fit attribute that can be added to the <mapbox> element, however it does not currently work with feature layers. I've created a new ticket for this at #35. Thanks for bringing this to my attention.