This change decouples the style-generation from the actual directives, meaning that the library is now composed of the following:

• Directives that respond to inputs and matchMedia events
• Style generation providers that return styles when triggered by directives

This allows for end-users or library authors to provide their own unique style generation (even by borrowing or extending our existing library code) for any directive. This is entirely non-mandatory for use of the BaseDirective, since the BaseDirective need not always use a de-coupled style provider to function.

The canonical example is the following:

@Injectable()
export class CustomStyleBuilder extends StyleBuilder {
  buildStyles(input: string) {
	return {
		'style1': 'value1',
	};
  }
}

@NgModule({
	...
	providers: [
		provide: <the style builder to orverride, e.g. FlexStyleBuilder>,
		useClass: CustomStyleBuilder,
	],
})
export class MyAppModule {}

Fixes #689