add option to disable auto assign

[nadar]

this closes #439

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[Antikon]

It seems to me that in this case (with disabled auto assign) in a CRUD list view we will get the raw value of a field. One of the points of SelectArray plugin is to dispay the "converted" data in a list view also.

Maybe it possible to make conversion on the frontend instead of the backend. We can pass once per page a conversion array, and the angularJs can do the rest. By logic, this plugin is not intended for the large amount of data, so the frontend conversion won't cost much.