Dummy component support

[bergie]

Right now we only support starting a network if all components used inside it are actually available and runnable.

However, there is certain amount of demand for having partially-implemented graphs, for example with Flowhub and top-down design (noflo/noflo-ui#266). In this case we'd want to instantiate "dummy" (no-op) components for the graph nodes that don't yet have an implementation, and otherwise run the network normally. Messages received by these no-op components could either raise an error, or be ignored.

Potential implementation for dummy components could be based on the MsgFlo foreign participant format where you can define things like ports.

[bergie]

We can use the # @runtime annotation to distinguish NoFlo dummies from MsgFlo foreign participants