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).

src/flake8/formatting/base.py

@@ -148,7 +148,7 @@ class BaseFormatter(object):
         """Handle logic of whether to use an output file or print()."""
         if self.output_fd is not None:
             self.output_fd.write(output + self.newline)
-        else:
+        if self.output_fd is None or self.options.tee:
             print(output)
 
     def write(self, line, source):

src/flake8/main/options.py

@@ -26,6 +26,7 @@ def register_default_options(option_manager):
     - ``--exit-zero``
     - ``-j``/``--jobs``
     - ``--output-file``
+    - ``--tee``
     - ``--append-config``
     - ``--config``
     - ``--isolated``
@@ -171,6 +172,11 @@ def register_default_options(option_manager):
         help='Redirect report to a file.',
     )
 
+    add_option(
+        '--tee', default=False, parse_from_config=True, action='store_true',
+        help='Write to stdout and output-file.',
+    )
+
     # Config file options
 
     add_option(