Set pre elements to overflow: visible

Some sites set them to auto, which messes up our cursor (and probably more)

lib/codemirror.css

@@ -175,6 +175,7 @@ div.CodeMirror span.CodeMirror-nonmatchingbracket {color: #f22;}
   color: inherit;
   z-index: 2;
   position: relative;
+  overflow: visible;
 }
 .CodeMirror-wrap pre {
   word-wrap: break-word;