Add default() to Path for fallback values (#7)

[1blankz7]

Closes #7.

Path::optional() (from #8) omits the key entirely when a value can't be resolved along the chain. This adds Path::default($value), which instead returns a fallback value and keeps the key present in the result set. default() takes precedence over optional() if both are chained.

$resolver = new QueryResolver(
    new Query('pilotName', (new Path())->get('pilot')->get('name')->default('Unknown pilot'))
);

$resolver->resolve($someShipWithoutAPilot);
// ['pilotName' => 'Unknown pilot']

• Added Path::default() with unit + functional tests
• Documented in README