Introduce ContentGetParserOutput hook

Introduce ContentGetParserOutput hook

This hooks allows extensions to override the normal model-specific
rendering of page content. A typical use would be to provide syntax
highlighting for pages that contain scripts. In that sense,
ContentGetParserOutput is a generalization of the old ShowRawCssJs
hook.

This, together with I76412f9d, is a (hopefully) fixed version of the
reverted Ibfb2cbefea44.

Change-Id: I979e2438689648ba4c959d8083197ef14ce524e8