Allow easier local dev with canary versions.

This allows specifying the Ember and Handlebars versions in the query string. Added to enable easier local testing against canary versions of Ember (just run `ember s --environment=production` in Ember repo) via:

/ember-performance/index.html?ember=http://localhost:4200/ember.prod.js&handlebars=http://localhost:4200/handlebars/handlebars.js