Make the default sections a configurable value

Previously it was hard coded to [public*, func*] but now we default to
that but make it configurable by the user with the
breathe_default_sections config variable.
Document breathe_default_sections config variable
Improve group content extraction

We account for the :sections: option when extracting the content from
the doxygen group when rendering in :content-only: mode. Previously it
wasn't being taken into account so we were displaying items which didn't
match the :sections: preferences.
Merge branch 'global'