v0.2.0

Second release of the tokio-task-manager crate.

• add shutdown_gracefully_on_ctrl_c method as an alternative to wait:
  • the difference with wait is that it will block on the CTRL+C (SigInt) signal,
    and once received (or until no open tasks are alive) it will start the graceful
    shutdown process automatically;
• add runnable TCP Echo Server example to showcase this new feature;
• improve documentation comments and add crate example;