[FR] Multiple actions support

[YBogomolov]

Motivation

Currently Notistack supports only one possible action, while sometimes it is useful to have multiple actions on the snack. Such actions may include a pair of buttons: one for closing the persistent snack, and the second one for displaying a modal dialog with error details.

Implementation idea

Remove <span> which wraps action in SnackbarItem, or check if action is actually an array and map through it.

[iamhosseindhv]

Thanks @YBogomolov. This is next on the list to be implemented.

[iamhosseindhv]

Fixed. Refer to #110 for more info.