Add Prompt Toolkit as an optional dependency

[corranwebster]

Prompt toolkit is a modern cross-platform Python replacement for GNU readline.

This PR add support for prompt toolkit if it is available, falling back to readline and/or standard input if it is not available. It also adds it as an optional install option via pip (ie. you should be able to install it via pip install adventurelib[prompt_toolkit]) and is added to the tox.ini for testing.

[lordmauve]

Sorry for the delay, wanted to try this out properly.

In principle this looks good - will help solve #16 and could go towards other features like tab completion.

However, it seems that in its default configuration, prompt_toolkit isn't set up for things like up/down to move through history. I think we should at least turn that on before merging.

[corranwebster]

Initially I was just going for basic arrow key support on OS X, but it looks like it is fairly straightforward to add history functionality as well. It does mean restricting to prompt_toolkit >= 2, but I think that's fine.