Support streaming outputs

[carolynvs]

Some outputs have particular value to the user as the bundle is executing, for example the logs or a hypothetical bundle' "progress bar". After discussing this in today's CNAB meeting, we think that we can use additional metadata on the outputs, and perhaps more well known output names/labels to indicate that an output should be watched by the driver and streamed back to the runtime.

• We need to do a bit more design work for this and probably write up a custom extension that bundle's can use to add this metadata.