Feature: Simplify usage by supporting new default loop. (#99 by @clue)

// old (still supported)
$stdio = new Clue\React\Stdio\Stdio($loop);

// new (using default loop)
$stdio = new Clue\React\Stdio\Stdio();

Improve code examples and documentation. (#100 by @clue and #98 by @PaulRotmann)

Use GitHub actions for continuous integration (CI). (#97 by @SimonFrings)