Allow for the syntax to be included in another

It is hard to include the Clojure syntax in another (for example, code
blocks in Markdown) because some regions use contains=TOP, which does
not work if there are other syntax rules.  Putting all top-level
elements in a cluster solves the problem.

[Committer's notes]

    This is a partial merge of
    https://github.com/guns/vim-clojure-static/pull/41
Generate clojureTop syntax cluster