Site prism

Adding support for SitePrism elements generation.

There was a redesign, the setup provides the context, which knows which builder to use to generate code snippets.
Also, elements have a prism_selector attribute, which is used by the Prism builder.

The only thing that I would like you to remove is this gem "terminal-notifier-guard" as it works only for Mac OS users. 

The other additions are awesome and I'm looking forward to merge.