Add support for multiple toasters

[timolins]

This PR implements the ability to have multiple toasters at once.

// Create a toaster with a custom id
<Toaster id="custom-toaster" />;

// Create a toast within the new toaster
toast.success('Nice toast!', {
  toasterId: 'custom-toaster',
});

<Toaster /> // Default toaster
<div>
  <Toaster
    toasterId="area1"
    containerStyle={{ position: 'absolute' }}
  />
</div>
<div>
  <Toaster
    toasterId="area2"
    containerStyle={{ position: 'absolute' }}
  />
</div>

Related issue

Closes #158