pip-compile --output-file test.txt also writes to stdout

[blueyed]

With pip-compile --output-file test.txt test.in the output should only be written to the file, and not to stdout additionally.
This happens through log.info (https://github.com/nvie/pip-tools/blob/a5ef99f320e1eb3841a8571131e1cfc8aa6ed4e4/piptools/writer.py#L96).

I think it should write to stdout only when - is used for --output-file (which would be a new feature also).

Related: #353.

Support for stdin/stdout was initially requested/discussed in #104.

[cjerdonek]

Issue #275 is also related.

[atugushev]

It writes to stderr since 4.0.0. Can be suppressed by --quiet flag.

[blueyed]

@atugushev Thanks!