keyTree.add(keys)

Adds items into the structure and returns the keyTree.

const keyTree = new KeyTree( [ Object, Object, Array ] );

function handler1() {}
function handler2() {}
function handler3() {}

keyTree.add( [ "click", "li", handler1 ] );
keyTree.add( [ "click", "li", handler2 ] );
keyTree.add( [ "click", "span", handler3 ] );

The keyTree data structure will look like:

{
    "click": {
        "li": [ handler1, handler2 ],
        "span": [ handler3 ]
    }
}