Add .onReady function

@harthur this change makes it so that the interface is a bit easier to work with.  After this change, to call setProjectToSinglePath, it would look like this:

    const {Itchpad} = require("itchpad");
    let itchpad = Itchpad(iframe);
    itchpad.onReady(() => {
      itchpad.setProjectToSinglePath("/some/path");
    });

What do you think?  Is there a better way to deal with this async stuff in this case (can't call setProjectToSinglePath until onLoad is finished).  I guess we could wrap the constructor inside of an exported function that returns a promise?