Add an option not to show guide-wide extra code

[felipecalvo]

When Show extra code is checked, the guide's extra code is shown first and then the exercise's extra code is appended to it.

A guide's extra code might be used in many exercises but not all of them, and showing all of it when it's not needed adds confusion to the exercise.

This guide could benefit from this feature; I'm leaving the link for future reference.

Issue might be related to #1125

[flbulgarelli]

I think we need a hide_guide_extra option here, that hides - but does still include - the guide extra code.

The steps required to do this are those listed in https://github.com/mumuki/mumuki-laboratory/issues/1125#issuecomment-494075296