Implement --extend-ignore

Refs #365

src/flake8/main/options.py

@@ -18,6 +18,7 @@ def register_default_options(option_manager):
     - ``--format``
     - ``--hang-closing``
     - ``--ignore``
+    - ``--extend-ignore``
     - ``--max-line-length``
     - ``--select``
     - ``--disable-noqa``
@@ -108,6 +109,13 @@ def register_default_options(option_manager):
              ' For example, ``--ignore=E4,E51,W234``. (Default: %default)',
     )
 
+    add_option(
+        '--extend-ignore', metavar='errors', default='',
+        parse_from_config=True, comma_separated_list=True,
+        help='Comma-separated list of errors and warnings to add to the list'
+             ' of ignored ones. For example, ``--extend-ignore=E4,E51,W234``.',
+    )
+
     add_option(
         '--max-line-length', type='int', metavar='n',
         default=defaults.MAX_LINE_LENGTH, parse_from_config=True,

src/flake8/style_guide.py

@@ -3,6 +3,7 @@ import collections
 import contextlib
 import enum
 import functools
+import itertools
 import linecache
 import logging
 
@@ -166,7 +167,10 @@ class DecisionEngine(object):
             self.selected + self.enabled_extensions,
             reverse=True,
         ))
-        self.ignored = tuple(sorted(options.ignore, reverse=True))
+        self.ignored = tuple(sorted(
+            itertools.chain(options.ignore, options.extend_ignore),
+            reverse=True,
+        ))
         self.using_default_ignore = set(self.ignored) == set(defaults.IGNORE)
         self.using_default_select = (
             set(self.selected) == set(defaults.SELECT)