Support layout-independent keyboard key identifier

[tbazin]

Hi! Thanks a lot for this library! 😃 Here's a PR to make it international! 🗺️

• Adds a layoutIndependentMapping option to use the layout-independent 'code' key identifier in keyboardEvents instead of the current (deprecated) 'keyCode'.
• Introduces additional tests with this setting enabled (pulled from the original QWERTY tests).

Fixes #9