all: add support for Angular 8

[christophercr]

I'm submitting a...

[ ] Regression (a behavior that used to work and stopped working in a new release)
[ ] Bug report  
[X] Feature request
[ ] Documentation issue or request
[ ] Support request => Please do not submit support request here, instead see https://github.com/NationalBankBelgium/ngx-form-errors/blob/master/CONTRIBUTING.md#got-a-question-or-problem

Current behavior

NgxFormErrors is compatible only with Angular 6 and 7, however Angular 8 has been released in May 2019 and many apps are now using the latest version.

Expected behavior

NgxFormErrors should also support Angular 8 so that any app using Angular can use it. This should be straightforward since this library is really lightweight and doesn't rely on any specific Angular API that is due to change in version 8 or even 9.

What is the motivation / use case for changing the behavior?

Potentially increase the usage of this library by any Angular application.

Environment

Angular version: 8.x