Allow string in `model`property from field configuration object

[benoror]

Since all my configurations come from a JSON backend, I would like to use a string in model property from field configuration object.

Originally I wanted it for field-groups, in order to support nested forms, but I guess it can be extended for any kind of field.

** Note: this is currently supported when specifing model as formState (#299)

JSBin: http://jsbin.com/bitoli/1/

[benoror]

Wouldn't key property for fieldGroup make more sense?:

JSBin: http://jsbin.com/tusubo/1/

[benoror]

Added both: #314

[kentcdodds]

Supported in 6.9.0