Implement --extend-ignore

Refs #365

docs/source/user/invocation.rst

@@ -107,6 +107,10 @@ And you should see something like:
       --ignore=errors       Comma-separated list of errors and warnings to ignore
                             (or skip). For example, ``--ignore=E4,E51,W234``.
                             (Default: E121,E123,E126,E226,E24,E704)
+      --extend-ignore=errors
+                            Comma-separated list of errors and warnings to add to
+                            the list of ignored ones. For example, ``--extend-
+                            ignore=E4,E51,W234``.
       --max-line-length=n   Maximum allowed line length for the entirety of this
                             run. (Default: 79)
       --select=errors       Comma-separated list of errors and warnings to enable.

docs/source/user/options.rst

@@ -56,6 +56,8 @@ Index of Options
 
 - :option:`flake8 --ignore`
 
+- :option:`flake8 --extend-ignore`
+
 - :option:`flake8 --max-line-length`
 
 - :option:`flake8 --select`
@@ -416,6 +418,37 @@ Options and their Descriptions
         ignore = E121,E123
 
 
+.. option:: --extend-ignore=<errors>
+
+    :ref:`Go back to index <top>`
+
+    Specify a list of codes to add to the list of ignored ones. Similar
+    considerations as in :option:`--ignore` apply here with regard to the
+    value.
+
+    The difference to the :option:`--ignore` option is, that this option can be
+    used to selectively add individual codes without overriding the default
+    list entirely.
+
+    Command-line example:
+
+    .. prompt:: bash
+
+        flake8 --extend-ignore=E4,E51,W234 dir/
+
+    This **can** be specified in config files.
+
+    Example config file usage:
+
+    .. code-block:: ini
+
+        extend-ignore =
+            E4,
+            E51,
+            W234
+        extend-ignore = E4,E51,W234
+
+
 .. option:: --max-line-length=<n>
 
     :ref:`Go back to index <top>`