Disable auto-initialization of ng-model from editor

This causes an issue when you have the following case:

- Parent scope A
   - Child scope B

in scope B, we put a <ui-codemirror ng-model="myobject.mycode" />

At template intialization, myobject is undefined.
myobject is loaded asynchronously using an AJAX request in scope A.

The auto-initialization of ng-model has the side effect of creating, in
scope B :  myobject { mycode : "" }

When the "correct" myobject appears in scope A, it is shadowed by the empty
one created in scope B.