`contextmenu` support and missing `contextMenu` event

[nuno-agostinho]

Hi, I have two issues related to context menu handling in phylotree.js:

1. Allow to show context menu using the contextmenu event (as asked by Allow menu to be shown on 'contextmenu' #446), even if not enabled by default.
2. Fix the contextMenu event emission: I was trying to do what I said by manually simulating a user click, but I do not think this event is emitted, so this does not work:

tree.on("contextMenu", (node, event) => {
    event.preventDefault();

    console.log(node, event);
	
	event.target.dispatchEvent(new MouseEvent("click", {
		bubbles: true,
		cancelable: true,
		view: window,
		clientX: event.clientX,
		clientY: event.clientY,
		screenX: event.screenX,
		screenY: event.screenY,
		pageX: event.pageX,
		pageY: event.pageY
	}));

    return false;
});

Thanks for this library!

All the best,
Nuno