Add UIActivityIndicatorView option

Requested in https://github.com/cruffenach/CRToast/issues/58

Only part i'm not a huge fan of is https://github.com/dmiedema/CRToast/compare/cruffenach:master...dmiedema:loading-indicator?expand=1#diff-5f3010153047668460039f76eed8df7cR1313 for stopping the spinner. If anyone has a better solution i'm 100% open to ideas.