Updates to style enqueuing, post types, templates

Apologies: I meant to create separate pull requests for these changes, but I messed up the branches.

The changes here include:

* Changing the hook for `enqueue_scripts` to make it easier to dequeue
* Allowing the path to Font Awesome to be filtered or killed entirely
* Allowing multiple post types to support the shortcode with a filter
* Allowing themes to supply template files
* Stopping "via" from appearing in tweets if no handle is set

Please let me know if you'd like these requests separated.