Support info and warning boxes.

Add hacks in the filters to detect these in the html, and to generate
suitable latex in place (via a placeholder).

Change-Id: I047bf9736e044324ec33eabe7e0ab5a37e1a4942
Parameterise out box creation.

Change-Id: I79909d16c879fcee3afa5143eda17fbbb0c1dda6
Tidy up naming to avoid confusions.

Change-Id: Ide69d066ecb4f5803c0dc54029fea3dfa16631c5