Dynasnips should be able to take arguments from the url

Wirhin the body of a dynasnip, I should be able to call something like

def handle(name=nil)
  name = arg(:name)
  # ...
end

And have the value in 'name' be set either via snips parameters:

{dyna my-name}

Or have the value come from the URL:

http://site/dyna?name=my-name

An example this would be useful for is paginating the 'kind' dyna for going back into blog history.