cpp11 0.4.4

• Davis Vaughan is now the maintainer.

• as_doubles() and as_integers() now propagate missing values correctly
  (#265, #319).

• Fixed a performance issue related to nested unwind_protect() calls (#298).

• Minor performance improvements to the cpp11 protect code. (@kevinushey)

• Silenced an unknown attribute warning specific to the Intel compiler
  (r-lib/systemfonts#98).

• cpp_register() gains an argument extension= governing the file extension of
  the src/cpp11 file. By default it's .cpp, but .cc is now supported
  as well (#292, @MichaelChirico)