Custom feddback on success / error

Hi everyone.
I’m using this plugin for the first time and it’s great because it leave freedom to add the right markup needed.
I actually need to change the behaviour of the response. I’d like to change the markup too and add some icons / transitions.
Now I found how to do that changing public.js in (line 163)

function addFormMessage(formEl, message) {
    var txtElement = document.createElement('p');
    txtElement.className = 'hf-message hf-message-' + message.type;
    txtElement.innerHTML = message.text;
    formEl.insertBefore(txtElement, formEl.lastElementChild.nextElementSibling);
}

but I don’t think is a good idea because when I’ll update the plugin all the changes will be lost.
Is there another, smarter, way to achieve that?
Thanks!