Check backwards compatibility with older Angular version

[f1ames]

Are you reporting a feature request or a bug?

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Since we developed the integration with latest Angular version, it's not clear which older versions are supported. The assumption was it should be Angular 2+, however it was never checked.

The compatibility with older versions should be checked and short section added to readme file (could be similar to what CKE5 have - https://github.com/ckeditor/ckeditor5-angular#supported-angular-versions).

[engineering-this]

Angular 6+

Works fine.

Angular 5

Throws an error, it can be fixed by changing

ckeditor4-angular/src/ckeditor/ckeditor.component.ts

Line 194 in 32f1249

constructor( private elementRef: ElementRef<HTMLElement>, private ngZone: NgZone ) {

to

constructor( private elementRef: ElementRef, private ngZone: NgZone ) { 

Reason is that elementRef isn't a generic type before Angular 6. However this change doesn't seem to break anything in newer versions. I'll provide PR.

Angular 4

Angular 4 has incompatible package meta-data format, so it won't work without separate package.