fix(ngList) Fix model -> view rendering when using a custom separator

This change adds a new ng-list-joined-by attribute to the ngList directive.
This allows you to specify a custom string to join elements of the model
array together which is useful when you are also using a custom separator.

Example:

    <input type="text" ng-model="model.values" ng-list="|", ng-list-joined-by=" | ">

This will split an input string such as "Cat | Mouse | Dog" into the array
["Cat", "Mouse", "Dog"] and vice versa.