Add highlighting of rule in build statements

Highlight the rule being used in a build statement.  Also add `.` to acceptable characters in a rule name and relax whitespace matching before the name.
I assume syntax highlighting is still fast enough even after adding the additional *s? (I'm not familiar with emacs' regex engine.)

You can run "python misc/write_fake_manifests.py /tmp/foo" to generate a bunch of .ninja files in /tmp/foo to check.

Other than that, looks fine, thanks!