Support "hidden" sources for examples

[wbamberg]

In MDN, live samples can include sources that are not displayed in the page but are still applied. For example, this is often used to have some basic CSS styling for examples.

stumptown should probably support these. The suggestion is to implement them by allowing the author to include files named "hidden.[css|html|js]".

Should authors be allowed to include hidden and non-hidden sources of the same type? If so, should there be a particular order in which they are stuck together?