Add Flags template

Flags is a typecons template for storing OR combinaton of enum values in a type safe fashion.

I think that the possibility to use bitwise operation on enums is a bit of a hole in D type system;
Flags provides a type safe alternative modeled after Qt's Q_FLAGS
The `Force` parameter is subject to boolean blindness; let's use a named flag.

(Also, the correct naming would be `force`, i.e. non-capitalized)