add compact setting for generated CSS
This commit adds a compact setting to compass' config, minifying the generated
CSS so each rule set occupies a single line, instead of the usual one declaration
on each line separated by tabs or spaces.
This isn't the most minified of settings: there is a "compressed" option that
takes the minification further by putting everything in a sole line. "compact"
is a less drastic option, which maintains some readability.
Follow up from comments on I0d312e93bb3615640ecc91462dfd6f96bde1bed7.