Add support for download artefacts

[tmcgilchrist]

Add artifacts support:

• I'm using magic-wormhole (https://github.com/magic-wormhole/magic-wormhole) to send file across from the workers to the host, but it's not very good because it's going through a public server so it cannot go in production (it's encrypted though)
• A better solution would be to add first-class support in ocluster