Added Emberman to extensions list, tagged Ember-related extensions

[This is a simple extension](https://github.com/minusfive/emberman) which packages [middleman-ember/](https://github.com/mrship/middleman-ember/) and [sprockets-handlebars_template](https://github.com/GutenYe/sprockets-handlebars_template), and configures them to work optimally with Middleman.

I've also been [working to migrate middleman-directory to a more modern Ember setup, using Emberman](https://github.com/minusfive/middleman-directory/commits/use-emberman). I'm close (WIP), but will submit for your review as soon as I feel it's ready.