Add a repl to web servers.

The socket for a specific server is named `www-<port>`. Each server
will log its socket file name at startup. You should run
`./node_modules/.bin/rc /tmp/repl/www-<port>.sock` to connect to the
repl. For dev servers, `npm run repl` will connect to the socket
named after the usual development port, as a convenience.
Merge pull request #439 from npm/ceej/replify

Add a repl to web servers.