Adds an "Additional builtins/globals" setting

This setting allows the user to specify custom symbols that will be recognized as builtins by pyflakes in every file of the project.

It is useful when the __builtin__ module is programmatically altered, for instance through gettext.install().

Note that this doesn't always behave as expected because of bug #1088.

This is a backport of pyflakes's [merge request 145125](https://code.launchpad.net/~pyflakes-dev/pyflakes/1107587-add-builtins/+merge/145125)

*See also the [original discussion on the bug I filled](https://github.com/ninja-ide/ninja-ide/issues/1087).*