Associate `for` attribute in label elements with aria-label with input elements

[ThomasBurleson]

Consider usage scenario of a table with checkbox columns:

<label for='<some-id>'></label>
<p> ipsum lorem </p>
<md-checkbox id='some-id' ng-model="isChecked"></md-checkbox>
<!-- or even using:
<md-input id='some-id' type=checkbox type='checkbox'>
-->

Since the md-input or md-checkbox element has an assigned id, first check for a md-label element with matching for attribute value before warning about the aria-label; otherwise inject the lookup value (if aria-label was not explicitly specified).

md-label - when a for attribute could potentially register the aria-label is a global lookup. So the input element search associated labels is NOT a DOM search.

[marcysutton]

@ThomasBurleson I made a test sandbox where we can try out attribute pairings. When we have a distribution channel for Material figured out, I'm moving these to Codepen. But, for now: http://plnkr.co/edit/WpVB5u8v6U2PiyOoMiNJ

In short, to associate a <label> with a non-native element like <md-checkbox>, we will have to do a few things:

• Include aria-labelledby="label-id" on the Material component to be labeled. See aria-labelledby doc
• Include a matching id on the label or other text element
• Enable checkbox/input toggling from click or keypress on label to mimic native behavior