Adds a assume_globals parameter to the Checker

This is used to provide additional globals/builtins that should be
assumed to exist in the code, even though no declaration or import can
be seen.

This is not unrelated to [ninja-ide's issue 1087](https://github.com/ninja-ide/ninja-ide/issues/1087). Some modules, **including the standard gettext module**, might add symbols to the builtin module, making this very useful.


I am not 100% sure this is the way to go, but an option somewhere to ignore missing globals would really be appreciated :-)