Add LESS support (.less)

Cheating slightly as it uses the CSS lexer, as pygments currently does
not have a dedicated less lexer. But I figure language recognition and
90% percent correct syntax highlighting is better than neither.

samples/Less/screen.less

@@ -0,0 +1,14 @@
+@blue: #3bbfce;
+@margin: 16px;
+
+.content-navigation {
+  border-color: @blue;
+  color:
+    darken(@blue, 9%);
+}
+
+.border {
+  padding: @margin / 2;
+  margin: @margin / 2;
+  border-color: @blue;
+}