Support decoding into OsStrings

[BlacklightShining]

From what I can tell, docopt only supports parsing arguments into Rust strings. This means that if a program uses docopt, it won't be able to take any arguments that aren't valid UTF-8, even if they may be otherwise perfectly valid (e.g. a path to a file on a *nix system).

[BlacklightShining]

Related to #135, I think: the ability to take paths as OsStrings would probably solve that, while avoiding ambiguity in interpreting whether a Vec<u8> is a u8 option that can be passed multiple times, or a not-necessarily-valid-UTF-8 string.

[BurntSushi]

You are absolutely correct. I was not particularly principled with this when building docopt, so it is quite possible that this could require some refactoring.