Support listening on a specific host in webapp

[dwarfcrank]

The webapp doesn't pass the --host flag to rackup on startup, which defaults to listening on localhost.

This is problematic when the application that wants to use the sandbox is on a different host (e.g. a Docker container in the case of tdd-pingpong), so a command line flag or a config value (like http_port is now) would be nice.