Support keypad in Linux

[tkzv]

The example does not work with keypad arrows and Enter in Linux. Could you add support for them? This requires changing 3 conditions in main.lua:

In function love.update replace
love.keyboard.isDown('up') with love.keyboard.isDown('up') or love.keyboard.isDown('kp8')
love.keyboard.isDown('down') with love.keyboard.isDown('down') or love.keyboard.isDown('kp2')

In function love.keypressed replace
key == 'enter' or key == 'return' with key == 'enter' or key == 'return' or key == 'kpenter'