Allow symlinking job outputs

[adamkewley]

Some project ideas involve generating files that are shared between jobs. This use-case would better-suit a symlinked approach because the outputs of an astronomy project can be large (>1 TB).

Ideal implementation:

• Add symlink: true to expected output
• Warn users if they try to symlink something inside the working directory and working directory cleanup is implemented (the cleanup will invalidate the symlink)

[adamkewley]

Dropped for 1.0.0 release, put onto wishlist