Tag toggling and tags method

This includes some additions I needed for a project using this library. Feel free to merge into master if you think they are suitable to be included there.

- New `tags` method: Returns an array consisting of the current tag objects in a tagify instance.
- Tag toggling: Tags can be toggled when clicked, which flips a boolean in the internal tag object and changes the tag style, customizable through CSS. This is disabled by default and enabled with the `toggle: true` option.

@ggoforth @boneskull