Generate stubs from process outputs

[bentsherman]

It should be possible to generate a "stub" script from the process outputs, since the purpose of the stub is essentially to generate some empty output files that match the output declaration in order to trigger downstream processes. This should eliminate the need to explicitly define a stub in most cases.

Some edge cases to consider:

• defining a variable (without def) in the process body and referencing it in the outputs
• defining an output glob
  • arity option could be used to generate a valid collection of files