Add --configuration cli option for xcodebuild call

You can now specify the Xcode configuration passed through to the xcodebuild command. Previously it was hardcoded to Debug.

If you specify a scheme, xcodebuild no longer defaults to Debug, since in most cases, the scheme will specify the desired configuration. It is possible to specify both a scheme and a configuration (though you should really be using schemes for this).

The default configuration is still "Debug" if you don't specify a scheme or a configuration.