Support functions as file plugins too

It is possible to write plugins which are only a function. At the moment they
are called on each line manually. This allows the function also to be called
on each file once. It works similar to creating the class and calling `run` on
it immediately. The plugin function needs to return a generator.

This is based on the original comment in the `FileChecker.run_ast_checks`
method, but slightly modified as the original comment would've called the
return of the function. But the function could return the reports directly.

src/flake8/checker.py

@@ -466,11 +466,13 @@ class FileChecker(object):
 
         for plugin in self.checks.ast_plugins:
             checker = self.run_check(plugin, tree=ast)
-            # NOTE(sigmavirus24): If we want to allow for AST plugins that are
-            # not classes exclusively, we can do the following:
-            # retrieve_results = getattr(checker, 'run', lambda: checker)
-            # Otherwise, we just call run on the checker
-            for (line_number, offset, text, check) in checker.run():
+            # If the plugin uses a class, call the run method of it, otherwise
+            # the call should return something iterable itself
+            try:
+                runner = checker.run()
+            except AttributeError:
+                runner = checker
+            for (line_number, offset, text, check) in runner:
                 self.report(
                     error_code=None,
                     line_number=line_number,