Background image position options instead of center

## Explain your use case
There are many cases where 'center center' is not ideal for background positions as defined in the background image directive. https://github.com/cloudinary/cloudinary_angular/blob/master/projects/angular-cld/src/lib/cloudinary-background-image.directive.ts

## Describe the problem you’re trying to solve
A way to provide an override without some css !important hack to control image position.

## Do you have a proposed solution?
Input param for background position, defaults to center

```
@Input('clbackgroundposition') background_position = 'center center';
...
this.renderer.setStyle(nativeElement, 'background-position', `${background_position}`);