parse optional cue identifiers

[internet-diglett]

Closes #2

&str.to_cue() now handles optional cue identifiers.

use title_parser::{CueTrait};

// with cue identifier
let text = "1 - Cue\n00:01:14.815 --> 00:01:18.114\n- I'm text for a cue\n- Me too!";
let cue = text.to_cue().unwrap();
assert_eq!(cue.text, "I'm text for a cue\nMe too!");

// without cue identifier
let text = "00:01:14.815 --> 00:01:18.114\n- I'm text for a cue\n- Me too!";
let cue = text.to_cue().unwrap();
assert_eq!(cue.text, "I'm text for a cue\nMe too!");