Add filter to printPublicPath

[derweili]

This PR adds a filter to the printPublicPath method.

This makes it possible to set publicPath to a custom URL e.g. a CDN as described in #14

Resolves #14

How to use this filter:

add_filter( 'wpackio_print_public_path', 'set_public_path_to_cdn', 10, 3 );

function set_public_path_to_cdn( $public_path, $appName, $outputPath ) {
	if($appName === 'myAppName') {
		return 'https://cdn.example.com/';
	}

	return $public_path;
}

[swashata]

Please make the suggested changes and we can version bump and release.

[derweili]

@swashata hi, would be cool if you could take a look at this. Would love to have this feature. I think all your suggested changes are implemented. If anything needs to be updates, just give me a shout.

[swashata]

Thanks @derweili this has been released.