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
Please break it into 3 lines:
````
}
else
{
````

(Yes I know it takes up a lot of lines, but them's the rules)