Support using validators without attr_literals feature

[dtolnay]

Obvious follow-up to #4 I guess.

I know this is more user-friendly: #[validate(range(min = 18, max = 20))]

But would it be possible to support this as well? #[validate(range(min = "18", max = "20"))]

And similar for the other validators. This would allow validators to be used on stable Rust 1.15.

It looks like this may be as simple as just extending the lit_to_int and lit_to_float functions to handle string literals.

[Keats]

Yeah, that's the easy solution I think

[Keats]

Tooks lots of time but it's in 0.2.0 (along with struct level validation)