Implement `--extend-select` command line argument following what was
done for `--extend-ignore` in !233. This option can be used to
selectively add individual codes without overriding the default list
entirely.
Addresses the remaining item of issue #1061.
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).
