Custom default value for skipped fields

[CreepySkeleton]

I propose support for explicit default values for skipped fields:

#[derive(StructOpt)]
pub struct Opt {
    #[structopt(long, short)]
    number: u32,

    #[structopt(skip = "key")]
    k: String,

    #[structopt(skip = &[1, 2, 3])]
    v: Vec<u32>,
}

The syntax is skip = expr, the field will be assigned with (expr).into() instead of Default::default().

[TeXitoi]

OK for such a PR