Allow filters to generate content for other filters

[denisdefreyne]

Currently, filters only generate one kind of output.

It would be useful to let filters also generate other kinds of output. For example:

• a Markdown filter could output HTML, but also a parsed representation. Another filter could then do something with that parsed representation, which would be faster (as it eliminates another parsing step).

• A Sass filter could generate CSS but also all the information needed to efficiently generate a sourcemap.

Ideally, the non-primary output content would be lazily-generated, i.e. only generated when another filter actually uses the data.

[gpakosz]

Well... filters being able to generate content for other filters...

I don't how what it means with respect to reps and the ability to write different snapshots to different files.

At least, what would be useful is to have a way to let filter pin some additional data to an item: for example a :mathjax filter would be able to "mark" an item with "hey I've found some LaTeX in there!" and later during the layout phase you can know "oh some LaTeX got found, let's link to the MathJaX css stylesheet"