(feat) Added mount! macro to build mounts.

[turnage]

I saw that #79 hasn't gotten attention in months and I also wanted this feature.

This adds a mount! like the router! macro from the router crate. Its only value is that I can write

mount!("/assets" => Static::new("target/assets"))

in places I previously had to write

{
    let mount = Mount::new();
    mount.mount("/assets", Static::new("target/assets"));
    mount
}

Rendered preview of docs: