First take on cursor position

This is an in-progress PR so there's something to talk about while I make progress. Things I can think of to worry about:

* How should this be tested?
* Anything different to do for signals received during this blocking call?
* https://github.com/erikrose/blessings/issues/65 says Sun needs to be treated differently, and that this won't work in Windows. What's the right way to detect these situations?

I'll keep working on these but would appreciate thoughts.
please conditionally check return value of keystrokes it not None, as you will be adding a ``timeout`` parameter. Make sure to mark the start-time and compute the remaining time for each call to inkey(), You will have to use some kind of '_timeleft' function to compute remaining time like inkey() does.