debugger: run last command on presssing enter

PR-URL: https://github.com/nodejs/node/pull/6090


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: bnoordhuis - Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Myles Borins <myles.borins@gmail.com>
Reviewed-By: Claudio Rodriguez <cjrodr@yahoo.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Fixes: https://github.com/nodejs/node/issues/2895