Custom container for tagsinput stuff & some behavior improvements

Hello!

We are using your plugin in [our project](http://papeeria.com). Thank you, it works fine.

Here is short description of this merge request contents:
* added `container` option to `tagsinput`. Now it's possible to pass a custom block for all tagsinput stuff. Useful for styling
* when you press backspace in empty input field - it removes previous tag and set tag name to the input field.