Options to select a single axis for the joystick

[Rubbonn]

Fix: #23

I just added an option "axis" which can be "x", "y" or false (default). This way you can choose if the joystick must be on single axis or both.

I don't know how you minified the js so i'll leave this task to you.

[cauafroes]

underrated pull request. really useful param

[bobboteck]

I am considering what solution to take, I would also like to create a different graphic