integration tests for piping human output to a file

[killercup]

• Integration testing is set up

  There are various option how to do it, e.g. using examples or subcrates. Will most likely benefit from using assert_cmd and/or escargot.

• Assert that piping to a file omits any color information

• Assert that piping to a file omits any interactivity

The 'duct' crate allows spawning a command and piping its stdout/stderr to a buffer/file. I'm not sure how realistic that is (setting TERM env vars and such), so we might have to just spawn a sh instead.