Overload nonblock semantics with a timeout value

[kmayer]

Useful if you have an application that would like to sleep at the end
of every loop. This will let it wait for & collect the keypress until the
sleep would have expired.

For example, this loop will sleep until approximately the start of the next
second on the system clock.

loop do
  draw
  now = Time.now.to_f
  codes = reader.read_keypress(raw: true, nonblock: (now.ceil(0) - now))
  command.interpret(codes) if codes
end

Describe the change

Overload the semantics of the nonblock so that it can take a timeout value as well as Boolean.

Why are we doing this?

I'm building an application that is like top: It updates the terminal screen at 1 second intervals, and
needs to listen to keyboard commands. I was having trouble capturing the key presses in my event loop.
I had a sleep in the loop, but then I noticed that Console#get_char# has a TIMEOUT` constant.

Benefits

This adds a useful feature for writing looping applications.

Drawbacks

Not supported in win_console.

This may not be the best approach for this type of application. I did a bunch of experimentation before I
got this to work to my satisfaction.

There are no tests around the nonblock option, and I'm not sure how to test a timeout option without
introducing something like the Timecop gem to the development environment.

Requirements

• Tests written & passing locally?
• Code style checked?
• Rebased with master branch?
• Documentation updated?
• Changelog updated?