New multiline tags system.

Instead of setting @multiline = true, the multiline tags now return
a tag containing {:doc => :multiline} from their #parse method.

Inside Doc::Parser the free text will only get appended to these
:doc=>:multiline tags plus a special :tagname=>:doc tag which grabs
all the documentation since the beginning of doc-comment up to the first
multiline @tag.

The Doc::Ast will later combine docs for classes and members, mainly
looking at the :tagname=>:doc, but also at :cfg and :property which are
a special case because they can contain subproperties, and also
:constructor which can be used as a splitter between class documentation
and constructor documentation.