Provide an op-out option

[khashayar]

There is this conflicting scenario of using another library (i.e: ng-bootstrap Typeahead), which is applying its own custom value accessor to the input field, in this case angular throws the following error:

Error: More than one custom value accessor matches form control with unspecified name attribute

Although it has been addressed in #7 but the fix neither covers all the cases nor provides and opt-out solution. As a result the need of a general op-out option seems to be necessary.