use yui-compress option

Author: lukeapage

lib/less/env.js

@@ -58,6 +58,7 @@
         'silent',      // whether to swallow errors and warnings
         'verbose',     // whether to log more activity
         'compress',    // whether to compress
+        'yuicompress', // whether to compress with the outside tool yui compressor
         'ieCompat',    // whether to enforce IE compatibility (IE8 data-uri)
         'strictMath',  // whether math has to be within parenthesis
         'strictUnits'  // whether units need to evaluate correctly