Use constant-face for :keywords

This uses constant-face for everything that looks like a keyword:
something that starts with a :, and is terminated by whitespace, or
something that needs to get highlighted another way (eg, parens).

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>
Merge pull request #2 from algernon/f/keyword-font-lock

Use constant-face for :keywords