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.
@smarterclayton  Because of the way this object is used, an anonymous field won't work because "len(util.StringList)" is a construct used elsewhere and that method requires a slice, not a struct.  One alternative is to make our own copy of StringList.  I think that makes sense if we expect more lists from command arguments, but copy-and-paste reuse feels like I'm fighting the language.  Any advice?