Introducing 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.

Change-Id: Ibfb2cbefea44eeee9f2a027f47e7721bf177ba0f