Allow disabling of highlight.js

[NullVoxPopuli]

I'd like to swap out highlight.js for Prism.js, as Prism has better syntax-highlighting / ASTs than highlight does.

I just need highlight disabled, and Prism can be loaded manually, and it's apis can be invoked manually if needed. :)

I see Highlight is used here: https://github.com/ef4/ember-code-snippet/blob/dd00ef16c6956e04c30869ece7818b311d7e6fa5/app/components/code-snippet.js#L5

could this be optional?