Allow implicit construction from empty_t

[usefulcat]

If flags(empty_t) were not declared explicit, I could do the following:

void f(int x, flags::flags f = flags::empty);

Instead of:

void f(int x, flags::flags f = flags::flags(flags::empty));

..which of course is repetitive and more verbose.

Not a huge deal but maybe worth changing unless there is a more important reason for it to be explicit.

Thanks for the library by the way, I find it quite useful!