Fix using multiple positions at the same time

[antriver]

Fixes #438

[zverev78]

It will create multiple <div>s with the same id. Not an ideal solution.
Possibly we should change containerId to options.containerId + '-' + options.positionClass when options.containerId is default (i.e. == getDefaults().containerId),
change in css all #toast-container to .toast-container
and add this css class in createContainer().