Better readline

[TimWhiting]

@shogo314

If you create a better readline, feel free to make a PR here in the unofficial community libraries so we can all benefit. We can provide feedback, and help, and then hopefully make a PR to the Koka std library at some point when it is polished. In the meantime, this provides a staging ground for creating and using potentially unstable code with no guarantees, and a way to play around with different apis prior to committing to something in Koka's core libraries.