Support suppressing stdout of each subtasks

[aengelberg]

lein monolith each already supports an :output option that multiplexes each subproject's output to a dedicated file on disk. That helps me pick apart the output of each test suite, but I'd also like to not print that output to stdout during the run, so I can focus on the high-level progress messages and end-of-run details telling me which project(s) failed. This behavior could be implemented by either:

• Change the behavior of :output so that it not only redirects the output but also suppresses it from the main process's stdout
• Supporting a distinct flag like :print-output false, then rename :output to :save-output, so the two options can be used in tandem to achieve the behavior I want