Added ',' thousands grouping to int.__format__. See PEP 378.

This is incomplete, but I want to get some version into the next alpha. I am still working on:
Documentation.
More tests.
Implement for floats.

In addition, there's an existing bug with 'n' formatting that carries forward to thousands grouping (issue 5515).
diff --git a/Objects/stringlib/stringdefs.h b/Objects/stringlib/stringdefs.h
index 946de05..1659315 100644
--- a/Objects/stringlib/stringdefs.h
+++ b/Objects/stringlib/stringdefs.h
@@ -24,5 +24,6 @@
 #define STRINGLIB_CMP            memcmp
 #define STRINGLIB_TOSTR          PyObject_Str
 #define STRINGLIB_GROUPING       _PyBytes_InsertThousandsGrouping
+#define STRINGLIB_GROUPING_LOCALE _PyBytes_InsertThousandsGroupingLocale
 #define STRINGLIB_TOASCII        PyObject_Repr
 #endif /* !STRINGLIB_STRINGDEFS_H */