Nullable additions

[dcdillon]

I would like to add two convenience member functions to Rcpp::Nullable<T> specifically:

T Nullable::as()
{
    return T(m_sexp);
}

T Nullable::clone() const
{
    return T(Rcpp:::clone(m_sexp));
}

These are analogous to the Rcpp::as and Rcpp::clone functions and just add convenience to the Nullable class.

Any objections?

[eddelbuettel]

That sounds pretty good, is un-objectionable as it also only 'grows' the interface not altering anything existing and completes the interface. PR away, if your fingers aren't frozen yet :)