Add "shift-select" a11y functionality

[jshcrowthe]

This is a PR to represent #204 - It is not complete yet (still needs tests, documentation, we need to decide whether or not this should be opt-in, or on-by-default, etc) but I wanted to have the code out there to play with.

I think it might be nice to get an a11y expert to take a look at this as there is some interesting use cases to be considered.

[blasten]

Thanks for the PR @jshcrowthe!

In https://github.com/PolymerElements/iron-list/issues/204 you said:

The thought was, because I am working with private API of the element, (e.g. this._getNormalizedItem(item), this._physicalItems.map(...), etc) that the functionality would be local to the element itself. However I was thinking that I should probably gate it behind a property to make it opt-in.

selectItem and deselectItem are public. Also, Templatizer defines modelForElement(target) where modelForElement(target)[list.as] is the object instance. I think with the current API, it should be easy to build a multi selection demo. wdyt?

[andreasanheuser]

Hey Josh, any update on this? Shift select is a feature that I am very interested in as well :-)