Merge branch 'master' into 'master'

Add --tee option to split report output stream.

The `--tee` option allows the linter report to be written to stdout, even
though it is being redirected to a file with the` --output-file` option.
This is useful if I want to store the report in a separate file for later
analysis but also be able to print the output on screen (e.g when running
in a CI environment).

See merge request !90