Backported merge request 145125 from pyflakes

This is a backport of a pyflakes patch adding a 'builtins' parameter to
Checker's constructor.
Adds an "additional builtins" setting

This uses the 'builtins' parameter for pyflakes' Checker to allow
the user to add custom symbols that will be considered builtin/global in
every file. This is useful for projects that add symbols to the builtin
module, such as projects using gettext.install().