Support for multiple custom actions in snacks

[YBogomolov]

See #107 for details.

My design decisions:

1. The only reliable way to ensure user is able to close the snack – to provide user's action with our internal onClose handler. This could be done via abstracting action into a generator function.
2. I also updated Redux example so you can see implementation from the user's perspective.
3. The old behavior is preserved, so no regression is introduced by this PR.

Notes: I had to create src folder in redux-example and move corresponding files into it so the example could actually work.

[YBogomolov]

@iamhosseindhv any thoughts on this? Does my design suit overall architecture of Notistack well?

[iamhosseindhv]

@YBogomolov Thanks for the PR. like idea of passing an action function in options parameter. (i.e. getAction in your implementation). But we need notistack to be as close as possible to material-ui snackbars, in terms of the API. So I try to not add any new getAction or onClickAction prop/options.

Anyhow, this feature is now in master and will be published within the next week.

We now have access to the key for a given snackbar which can be used to dismiss the snackbar:

props.enqueueSnackbar('I love snacks!', {
    action: (key) => (
        <Fragment>
            <Button onClick={() => alert(`I belong to snackbar with key ${key}`)}>MyAction</Button>
            <Button onClick={() => props.closeSnackbar(key)}>Dismiss</Button>
        </Fragment>
    ),
    // OR: 
    action: (
         <Button onClick={() => alert('I don\'t care which snackbar I belong to')}>MyAction</Button>
    ),
 });

[YBogomolov]

@iamhosseindhv Sorry, but I have to reopen this issue. Due to incorrect typings it's impossible to use this feature in TypeScript. I'll try to come with a PR as soon as possible.

[YBogomolov]

@iamhosseindhv Please see #113. I would be grateful if this issue is addressed ASAP.