nice progress bars for docker pull download

In #61 custom pulling via dockerode was added which required our own progress display in the console (since it's now longer done by `docker run` for us). The current implementation here:

https://github.com/apache/openwhisk-wskdebug/blob/d44bfb46c3c9254929a71c24bd3635aa9f350fe7/src/invoker.js#L191-L236

is ok-ish, but a nice visual progress bar would be better.

Some links:
- https://github.com/AndiDittrich/Node.CLI-Progress/blob/master/examples/example-visual.js
- https://github.com/AndiDittrich/Node.CLI-Progress/blob/master/lib/format-bar.js
- https://github.com/AndiDittrich/Node.CLI-Progress/blob/1314b5b4965184d0d7a23617aaf2d2bf052b9544/lib/options.js#L48