"-(tab)string" is incorrectly parsed

[ExpHP]

Found this while modifying the tests for #148. The current implementation incorrectly parses examples 6.2 and 6.3 in the spec.

#[test]
fn ex() {
    use yaml_rust::YamlLoader;

    let docs = YamlLoader::load_from_str("-\tb").unwrap();
    assert_eq!(docs[0].as_vec().unwrap()[0].as_str().unwrap(), "b");
}

thread 'ex' panicked at 'assertion failed: `(left == right)`
  left: `"\tb"`,
 right: `"b"`', src\main.rs:100:5