Remove support for old categories.json file syntax.

The old syntax has been deprecated for a while - time to remove
support completely.
Add Tag#repeatable to multiple times occuring tags.

Most tags don't make sense to be used multiple times - like all
boolean tags.  So I introduce a #repeatable field which must
be set to true for all tags that can be used several times
within one doc-comment.

When tag not marked as #repeatable, generate a warning
of type :tag_repeated.
Remove repeated tag warning from @since.

This case is now covered with the #repeatable field and gets
generated for the @since tag automatically.