Simplify usage by supporting new default loop

[SimonFrings]

This changeset simplifies usage by supporting the new default loop.

// old (still supported)
$launcher = new Clue\React\Shell\ProcessLauncher($loop);

// new (using default loop)
$launcher = new Clue\React\Shell\ProcessLauncher();

Builds on top of reactphp/event-loop#226, reactphp/event-loop#229, reactphp/event-loop#232, reactphp/child-process#87 and reactphp/stream#159

[clue]

@SimonFrings Thanks for the update, changes LGTM!