Add the `prefix` attribute

[chanman3388]

The idea here was to allow the addition of the prefix attribute such that when displaying the enum variant that said variant will have the prefix. This should be exempt from other formatting. Example usage:

use strum::{Display, EnumString};

#[derive(Debug, EnumString, Display)]
#[strum(prefix = "prefix")]
enum Thing {
    Test,
}

fn main() {
    println!("{}", Thing::Test);  // prints "prefixTest"
}

Could possibly add some kind of skipping mechanism for individual variants.

[Peternator7]

Looks great! Merged