Add support for mini-fab button as trigger

[ngehlert]

Currently if you use mat-mini-fab as speed-dial-trigger no button will show up. Would be great if you could support mini-fab button as well there. If I have time in the next weeks I can also have a look at the code and maybe make a PR

[PowerKiKi]

I am not entirely sure that a mini-fab as a speed-dial-trigger is supported by the spec. But I certainly cannot find something that explicitly forbids it.

So if you create a reasonable PR, including tests and demo, I'd probably merge it.