Support #[from] on an Option field

[dtolnay]

This allows ? to work with error types that only sometimes have a source (as seen in #1).

use std::io;
use thiserror::Error;

#[derive(Error, Debug)]
#[error("...")]
pub struct Error {
    #[from]
    source: Option<io::Error>,
}

fn main() -> Result<(), Error> {
    io::copy(&mut io::empty(), &mut io::sink())?;
    Ok(())
}