Allow output of measurement data to stdout again

[UsualSpec]

84c59ab introduced the -O flag which allows writing all output to a file. Now all output goes to stderr by default.
I believe that measurement outputs are not errors and should therefore - like before - still go to stdout (if the -O flag is not set).

In our project, we get data from stdout of pinpoint (ref: https://github.com/nsg-ethz/autopower/blob/cdb4c9212433088ddd614a17a9a198c33e689427/client/client.cc#L201) and reverted some changes as quick fix:
nsg-ethz@23d1aea nsg-ethz@dc8cefa

This is not ideal as it diverges from upstream, and I'd like to propose some changes to output handling:

As easy fix: allow passing stdout to the -O flag to print to stdout instead of stderr. This will then disallow writing to a file called stdout - but this can be documented.

As more involved, but potentially cleaner fix: to introduce an error output stream which - if -O is set - writes to the file together with the output_stream, else just goes to stderr, while output_stream goes to stdout.

What do you think?