Define a processing API for the <map> element

[prushforth]

In addition to WebIDL, the HTML spec often includes a prose description of the API immediately after the definition of the API, like this: http://www.w3.org/TR/html5/embedded-content-0.html#htmltrackelement for the track element. The same should be provided for the map element,
below the WebIDL, here: http://maps4html.github.io/HTML-Map-Element/spec/#todo3

[prushforth]

There is some work going on over here to help define the capabilities required.

Not only does the <map> element require an API, but also the <layer> element, and probably all the MapML elements should be considered from this perspective, too.