Implement multiple attribute

Needed by montagejs/matte#27.

abstract-select.js currently doesn’t set element.multiple attribute.

I implemented 'multiple' that was used by montagejs/native. Later I realized that abstract-select already has 'multiSelect' property which however doesn’t change element.multiple.

Should I keep only multiSelect and make it change element.multiple?  
If so, should it change element.multiple in abstract-select.js or in Matte & Digit? I’m thinking the former to avoid code duplication.