repl: add support for custom completions

Allow user code to override the default `complete()` function from
`readline.Interface`. See:
https://nodejs.org/api/readline.html#readline_use_of_the_completer_function

Ref: https://github.com/nodejs/node-v0.x-archive/pull/8484

PR-URL: https://github.com/nodejs/node/pull/7527


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Lance Ball <lball@redhat.com>