Get dependencies from an htmlwidget object

[cpsievert]

This just makes it a bit easier to obtain htmlwidget dependencies. For example, this allows me to do:

getDependency(leaflet::leaflet())

Rather than having to know the "name" of the widget

getDependency("leaflet", package = "leaflet")

[ramnathv]

This looks good @cpsievert. Let me test through a couple of scenarios before I merge it in.