efp.matrix: implement lrvar and vcov parameters

@appelmar Could you check this part? Achim implemented two optional parameters to efp[.formula], could this be ported to the matrix function as well? Or maybe we should merge the two functions so that changes like that are easier to integrate?

_Originally posted by @GreatEmerald in https://github.com/bfast2/strucchangeRcpp/pull/29#discussion_r584194275_