add flag to configure CORS

This flag enables configuration of the CORS allowed origins on the command line.  It enables functionality already present in kubernetes proper.
Origin uses spf13/cobra, which in turn uses spf13/pflag as it's flag library instead of the native flag  library.  Kubernetes uses the native flag library.  These are mostly compatible, except when you're trying to add a custom flag type.  In that case, the flag.Value type from native flag (https://godoc.org/flag#Value) is missing the "Type()" method that pflag.Value requires (https://godoc.org/github.com/spf13/pflag#Value).  Without inheritance to extend the Kubernetes StringList and without the ability to attach functions to structs defined in other packages, I don't know of a clean way to use the StringList.  Suggestions?