Implement no stdout flag

[krs85]

Some benchmarks (just pash, I think, actually) have funky pipe usage that doesn't play nicely with our stdout handling. Currently, we have an implementation for this in the master_no_stdout branch, but that branch is a bit behind master at this point, and having a flag would make life easier.

• Implement flag.
• Test that pash benchmarks still cache and skip properly.