Add ability to check if a file is ignored inline

Check for ``# flake8: noqa`` lines inside a file.

flake8/checker.py

@@ -291,6 +291,9 @@ class FileChecker(object):
 
     def run_checks(self):
         """Run checks against the file."""
+        if self.processor.should_ignore_file():
+            return
+
         try:
             self.process_tokens()
         except exceptions.InvalidSyntax as exc:

flake8/processor.py

@@ -1,6 +1,7 @@
 """Module containing our file processor that tokenizes a file for checks."""
 import contextlib
 import io
+import re
 import sys
 import tokenize
 
@@ -41,6 +42,8 @@ class FileProcessor(object):
     - verbose
     """
 
+    NOQA_FILE = re.compile(r'\s*# flake8[:=]\s*noqa', re.I)
+
     def __init__(self, filename, options):
         """Initialice our file processor.
 
@@ -269,6 +272,19 @@ class FileProcessor(object):
         """Read the lines from standard in."""
         return utils.stdin_get_value().splitlines(True)
 
+    def should_ignore_file(self):
+        # type: () -> bool
+        """Check if ``# flake8: noqa`` is in the file to be ignored.
+
+        :returns:
+            True if a line matches :attr:`FileProcessor.NOQA_FILE`,
+            otherwise False
+        :rtype:
+            bool
+        """
+        ignore_file = self.NOQA_FILE.search
+        return any(ignore_file(line) for line in self.lines)
+
     def strip_utf_bom(self):
         # type: () -> NoneType
         """Strip the UTF bom from the lines of the file."""