Allow taking an optional dependency on std to directly output to a String

[khuey]

It would be nice if it were possible to output to a String directly, rather than having to do a dance with format! and the Display impl. I assume the reason this is not currently possible is because this crate is no_std.

[alexcrichton]

Seems plausible to me! Note that you can also use .to_string() as well, although that's still not as ergonomic as getting a String directly

[khuey]

Yes, I should have mentioned that. to_string (which comes from std's impl ToString for T where T: Display) doesn't allow specifying the alternative formatting (i.e. no hashes) argument though.