This is a follow-on from #2286, which needed to add a stopPropagation call for the time being to avoid triggering a ripple on chip elements when a specific icon within the element is clicked.

It should be possible to suppress the ripple for particular child elements to avoid the need for stopPropagation. This would ostensibly be enabled via an optional setter/API on the ripple component / foundation that receives a selector; if present, potential ripple activation events would first check whether the event target matches the given selector (queried against the root element that MDCRipple was instantiated on).

This should have no overhead for the overwhelmingly common case where this isn't used at all.

(Note that this is specifically for child elements that don't already have ripples of their own, which is already prevented in #2123 and #2178.)