Add a Text Editor for Plinth to edit configuration files

Description of the project: Plinth is a web interface to administer the functions of the FreedomBox, working very well so far and out of the box for newbies. However, for some advanced user we need configure some application or system directly, so that we want to edit configuration file (such us /etc/privoxy/config for Privoxy configuration), some times it is hard to edit config file via SSH, so a web-based editor is better. It is necessary for Plinth to adding a text editor, not only a textbox but a syntax highlighting enabled editor. The mission is make it easy to edit config files and make it alive as save it.