Implement paper-type-ahead element.

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From @djanix on October 15, 2015 21:20

Example: if we want to reproduce something like this: https://twitter.github.io/typeahead.js/examples/

Right now, we can't keep the dropdown open while typing in another input. Maybe we could choose on wich input put the focus on when the dropdown is opening.

Copied from original issue: PolymerElements/paper-dropdown-menu#60

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My two cents: we need to implement type-ahead as a separate element. I'm going to move this issue to paper-elements.