Use all Solarized highlight colours

Added green and cyan.  Originally not included so as not to clash with
highlighting of keywords in Lisp, Scheme and Clojure.  Maintaining a
gradation of contrast and colour value is now the preferred goal.

Closes #17 (https://github.com/amdt/vim-niji/issues/17).
Add support for Solarized's degraded colour support in the GUI