Adds an option to disable the range API.

This is related to my Github Issue on Ember.JS regarding the performance of the range API:

https://github.com/emberjs/ember.js/pull/3110

The range API seems to be slower across the board compared to Metamorph.js' alternative implementation. My initial PR to Ember disabled it by default, but I suspect this is not as sane a default in metamorph itself.

This adds the ability to set `ENV.DISABLE_RANGE_API = true` and have Metamorph.js not use the range API, regardless of whether the browser supports it.

If this is accepted I will revisit the Ember PR to update to the latest Metamorph.js and to set the environment variable if not previously set so it is disabled by default in Ember for the performance improvement.