Allow "-xml" to pass through, similar to "--js-modern" and pass-through flags

If you need to generate XML type information for your project (like you want to use haxedoc or chxdoc to generate documentation), pass "-xml (file)" on the command-line.
Reverted previous change, restored class "-xml" target flag functionality in the new command-line tools.

This system does not require a path, and creates a standard system so types can be pulled from multiple target platforms.

If a specific XML path is needed, use <haxeflag name="-xml custom/path.xml" /> to customize output further.