[New library] Constable - A simple CLI shell for Arduino

I wrote a shell to interact with Arduino over a serial connection a while ago, and figured I should make it available to everyone. It allows you to define small functions that are called whenever you enter there name, and anything after that is parsed as arguments using spaces as a separator. It relies on btrstrlib for string handling, and I've included it in the repository.

It could use a lot of cleaning up, however I hope the examples included will be enough to allow someone to use it right away.

It's hosted on Github here:

Let me know what you think!