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:

ember-code-snippet/app/components/code-snippet.js

Line 5 in dd00ef1

var Highlight = self.require('highlight.js');

could this be optional?