Add sortablejsContainerClass input to the sortablejs directive

[darroshi]

Simple proposal to make sortablejs usable on the material components that wrap their elements, such as mat-chip-list.

This fixes issue #60.

[smnbbrv]

Hi,

thanks thats a good idea. I had a similar one, but with selector instead of class because the class might not always be present.

Another point is that the element will not be created yet in ngOnInit but in ngAfterViewInit. And finally all this needs to be really tested.

[smnbbrv]

implemented as sortablejsContainer in 3.1.0