Update CodeMirror dependency to version 5.8

This will add some features, especially to the search addon, and solve some issues with the addon currently used :

• (v5.5) Define a findPersistent command in the search addon, for a dialog that stays open as you cycle through matches
• (v5.6) The search addon now recognizes \r and \n in pattern and replacement input
• (v5.7) Make the closebrackets addon select only the wrapped text when wrapping selection in brackets

Full list of change between 5.1 and 5.8 : http://codemirror.net/doc/releases.html