Add spellcheck in strings.  Particularly helpful for docstrings.

Spellcheck is currently only enabled for comments from issue https://github.com/guns/vim-clojure-static/issues/10 .  Most of my spelling errors happen in docstrings, though.  This patch adds @Spell to all clojure strings.  I don't think there is a distinction between docstrings and regular strings in the syntax config, so this enables spellcheck on all strings.

Let me know if you have any questions.  Thanks.