Fix: Disabled fields get marked as invalid

[kingcody]

Only mark enabled controls

Supporting doc: https://angular.io/api/forms/AbstractControl#disable

Disables the control. This means the control will be exempt from validation checks and excluded from the aggregate value of any parent.

[kingcody]

@third774 https://github.com/kingcody/ng-bootstrap-form-validation/tree/1.x.x is a forked branch of 1.0.10 with this fix. Would you be willing to pull this branch in and release a 1.0.11 version for ng{2,4} based libraries?

[third774]

I'm going to branch off the 1.0.10 tag and cherry pick this commit and release so we have it here in the main repo. Thanks!

[third774]

https://github.com/third774/ng-bootstrap-form-validation/releases/tag/v2.0.1 and https://github.com/third774/ng-bootstrap-form-validation/releases/tag/v1.0.11 have been published.