Allow using noflo.asCallback with a graph instance

[bergie]

Right now, the noflo.asCallback embedding API only works with registered components or subgraphs.

To make it more flexible (as requested in #582), this PR adds a way to call it with a noflo.Graph instance as well.

So, you can either:

const wrapped = noflo.asCallback('myproject/Component');
wrapped(inputs, (err, outputs) => {});

or:

noflo.graph.loadFBP(`SOME FBP DATA`, (err, graph) => {
  const wrapped = noflo.asCallback(graph);
  wrapped(inputs, (err, outputs) => {});
});

Obviously the NoFlo graph can be constructed in whatever way... by loading JSON or FBP, or by using the graph API directly. However, it must have exported ports for this to work.