Redirect progress printing to env.err

RJ and I are working on printing progress indications during torrent creation. We're using https://github.com/mitsuhiko/indicatif and printing spinners and progress bars directly to stderr. We should instead redirect the output to `env.err`.

It looks like indicatif doesn't support this, but I opened https://github.com/mitsuhiko/indicatif/issues/153 to ask, just in case.

If it isn't supported, we should open a PR to add that feature, and modify intermodal to use it.