[Request] Add hover effect for toast-close class for better visibility on desktop

[GeorgeFlorian]

Hello.
First of all I have to say that this is a great library. I am using it on an ESP32 and it works flawlessly.

What do you think about adding a hover effect for toast-close (the X button) for a better visibility on Desktop.
I am currently using this in my implementation:

.toast-close:hover {
    opacity: 1;
}

Thank you.
Have a nice day.

[GeorgeFlorian]

Made a pull request: #87