[show-hint addon] Give active hint color more precedence

To make it easy for client code to color completions without clashing
with the selected styling.

Author: marijnh

addon/hint/show-hint.css

@@ -32,7 +32,7 @@
   cursor: pointer;
 }
 
-.CodeMirror-hint-active {
+li.CodeMirror-hint-active {
   background: #08f;
   color: white;
 }