allow spaces before/after braces for imports

[Luro02]

I personally really like to write imports like this

use std::io::{ Read, Write, Seek };

instead of

use std::io::{Read, Write, Seek};

so I'd really like to have an option to format imports like this with rustfmt :)

[eclipseeer]

Would be nice to have this option, coz it looks more readable as for me.