Add the possibility to use with multiple select elements

@DieterHolvoet to clarify this: You mean tagify should also work as better ux replacement for
<select multiple></select>
right?

Would you expect that as global replacement or as a widget to select on these fields?

Thanks @DieterHolvoet +1 on both! :)

Pushed work in progress to issue fork. Current approach:

• Use Select form element as base for new TagifySelect form element.
• Attach tagify to input HTML element with dropdown which copies options and default value from select HTML element.
• Update selected values via add and remove Tagify events.
• The value callback then is the same as for a select form element.

Some todos:

• Handle single value setting in form element. At the moment, multiple values setting is used.
• Check edge cases for no options, identical options text and identical options keys.
• Check hierarchical options.
• Check if JavaScript can be simplified.
• JavaScript linting.
• Add form element description.
• Work on form widget.
• Add tests.

And here is a small demo.

Update from #9

Pushed work in progress to issue fork. Current approach:

• Handle single value setting in form element. At the moment, multiple values setting is used.
• Check edge cases for no options, identical options text and identical options keys.
• Check hierarchical options.
• JavaScript linting.
• Add form element description.

Some todos:

• Check if JavaScript can be simplified.
• Work on form widget.
• Add tests.

Update from #15

Pushed work in progress to issue fork. Current approach:

• Handle single value setting in form element. At the moment, multiple values setting is used.
• Check edge cases for no options, identical options text and identical options keys.
• Check hierarchical options.
• JavaScript linting.
• Add form element description.
• Check if JavaScript can be simplified.
• Work on form widget.

Some todos:

• Add tests.

It will be included in release 1.2.15