X or Y single axis movement options with optional direction arrows

[kenloveday]

Added 3 joystick options:

• axisOnlyX {Bool} - Stick only moves in the X direction (y is set to 0)
• axisOnlyY {Bool} - Stick only moves in the Y direction (x is set to 0)
• internalDrawArrows {Bool} - Draws X and/or Y arrows on the centre of the Stick (if axisOnlyX=true only X arrow is draw, if axisOnlyY=true only Y arrow is draw)

Added example joy-axis.html

Updated README with new (optional) parameters and put description in 'Tips & Tricks' section.

[kenloveday]

This pull request is similar to the pull request by @Rubbonn but also addresses @bobboteck wish that this should have a different graphic by drawing (optional) arrows on the stick. See example joy-axis.html