This feature moves RDoc::Options#visibility= out to a separate method
and takes a synonymous argument of :all for :private.

Since :private is not an obvious name for documenting all methods of
this visibility, we provide this optional synonym. Also, this is similar
to how we can pass "--all" when parsing command-line options.
Merge pull request #276 from zzak/rdoc_options_visibility_setter_accept_all

Allow :all for RDoc::Options#visibility= as synonym for :private