Support for customized placeholders and relative time on comments

[rickwargo]

Thanks so much for this - it's been super helpful. I've added new functionality to enhance your package.

1. Support for custom placeholders - added messagePlaceholder and replyPlaceholder to make it easier to update
2. Comment date/time stamps - added a timestamp property to the message and its replies, using react-relative-time to display when created (and automatically set the creation time
3. Added a command line option to npm run start to support the legacy openssh library that is used by one of the other libraries