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.

Change-Id: Ice1dac49e1ea9bc05c4c6d3cf6d7201681954b67
Merge "add compact setting for generated CSS"