Support multi-doc deserialization

[dtolnay]

use anyhow::Result;
use serde::Deserialize;
use serde_yaml::Value;

fn main() -> Result<()> {
    let input = "---\nk: 107\n...\n---\nj: 106\n";

    for document in serde_yaml::Deserializer::from_str(input) {
        let value = Value::deserialize(document)?;
        println!("{:?}", value);
    }

    Ok(())
}

Closes #175.