fix negative offset for --move

src/main.rs

@@ -68,6 +68,7 @@ fn main() {
                      .short("m")
                      .long("move")
                      .value_name("OFFSET")
+                     .allow_hyphen_values(true)
                      .help("Move all subtitles in time (e.g 12:00.15 or -0:44)"),
             )
             .arg(clap::Arg::with_name("automove")