Added support for enabling mix manifest

[jaxwilko]

This PR adds support for enabling the Mix manifest, see Laravel Mix Asset Versioning.

The change here uses the existing options method provided by Mix to override our default behaviour.

// Enable default manifest
mix.options({
    manifest: true,
});

// Disable manifiest (default behaviour)
mix.options({
    manifest: false,
});

// Set a custom path for the manifest
mix.options({
    manifest: 'assets/manifest.json',
});