Allow linking files instead of copying

[almutlue]

Problem:
Sometimes espec. for references the user does not want to copy a file into the object, but only link to a file

Proposed solution:
Add an option to link files external to the object
modos add --external --source http://localhost/s3/reference/ref_genome.fa data/ex data

To consider:
Modos won't be self-contained any longer, if we allow external linking. This is a trade-off between storage space and reusability.