Update for angular 8 support

Bug description:

Add support for Angular 8. The current version works, but npm warns.

npm WARN @ng-bootstrap/ng-bootstrap@4.1.3 requires a peer of @angular/common@^7.0.0 but none is installed. You must install peer dependencies yourself.
npm WARN @ng-bootstrap/ng-bootstrap@4.1.3 requires a peer of @angular/core@^7.0.0 but none is installed. You must install peer dependencies yourself.
npm WARN @ng-bootstrap/ng-bootstrap@4.1.3 requires a peer of @angular/forms@^7.0.0 but none is installed. You must install peer dependencies yourself.

Versions of Angular, ng-bootstrap and Bootstrap:

Angular 8.0.0 ng-bootstrap: 4.1.3 Bootstrap: 4.3.1

[maxokorokov]

Thanks for reporting, we’ll be releasing the 5.0.0-rc.0 soon with:

• minimal required deps on Angular 8.0 and Boostrap 4.3
• Toasts and some minor features from Bootstrap introduced since 4.0

Can’t promise ivy support right from the beginning though.

[mendeza]

Well, your projects likely have tens, dozens, or scores of dependencies used in concert with Angular. Would you ask the same of them? Also, arguably there are two equally defining dependencies for this project: Angular and Bootstrap. Why privilege one over the other?

If it really was desired to have a scheme that reflected version support of both these pillars, it might be possible to abuse npm prerelease tags for this purpose, but that’s probably a really bad idea.