Implement an auto mode for artifact linking

[ctrueden]

Currently, linking can be configured as either hard, soft or none, with hard being the default. It would be nice to add a new mode, auto, that tries to use the best available successful form of linking (probably in the order hard, soft, copy).

• Python (as of Fix python jar linking #26)
• bash

[hanslovsky]

#26 fixed this only for Python jgo but we would like to see this in bash as well. Will close once implemented in bash.