Untrimmed key combination string silently causes issues

[SneakySensei]

If you pass an untrimmed string to any of the combo registration functions, it splits that without trimming which results in an invalid key combination.

Eg:

listener.simple_combo(" r", function() {
    console.log("This function is never called");
});

This happens because the combo " r" gets split into ["", "r"] which is not a valid combo. The library should trim the combo before passing it to the split function.

[SneakySensei]

I can take this up if this is a valid fix. @dmauro