Enable keyboard support with explicit config option input keyboard

The keyboard listening code is taken from [Kivy example][1].

I don't have any touch device, so having keyboard support is better for me to play with. Mouse is fine, but it'd be nice to have keyboard support. I think if request keyboard on all device might not be a wise option, that's why I choose this config approach:

```
python FlappyBird.py -c input:keyboard:True
```

If there is a way to check the current environment's keyboard information, if it's virtual, then don't enable by default, if it's not, then request the keyboard. That might be a better way in my opinion, but I don't know how to do it.

[1]: http://kivy.org/docs/api-kivy.core.window.html