Add an engine to run a workflow on an existing instance

[bwlang]

This could be done via the api...
(create history, upload data, run workflow)

but planemo run --engine sounds so easy.

[jmchilton]

This works now and there is a test case showing off different ways to use it here https://github.com/jmchilton/planemo-workflow-test-template. Thanks @bwlang.