[D] Refacto using CodeMirror contructor !
No more using [`CodeMirror.fromTextArea`](http://codemirror.net/doc/manual.html#fromTextArea) to create the CodeMirror editor.
Direct copy of the original element attributes to the created CodeMittir editor
[D] Flatten tests
[D] Initialise the value with the element content
[D] Update demo
[D] Update README