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).

docs/source/user/invocation.rst

@@ -125,6 +125,7 @@ And you should see something like:
                             available to use. (Default: auto)
       --output-file=OUTPUT_FILE
                             Redirect report to a file.
+      --tee                 Write to stdout and output-file.
       --append-config=APPEND_CONFIG
                             Provide extra config files to parse in addition to the
                             files found by Flake8 by default. These files are the

docs/source/user/options.rst

@@ -563,6 +563,26 @@
         output_file = output.txt
 
 
+.. option:: --tee
+
+    Also print output to stdout if output-file has ben configured.
+
+    Command-line example:
+
+    .. prompt:: bash
+
+        flake8 --tee --output-file=output.txt dir/
+
+    This **can** be specified in config files.
+
+    Example config file usage:
+
+    .. code-block:: ini
+
+        output-file = output.txt
+        tee = True
+
+
 .. option:: --append-config=<config>
 
     Provide extra config files to parse in after and in addition to the files