Add a progress bar showing the time until the toast closes

[konclave]

Implemented the progress bar that shows the time until the toast closes (#94)

How to use:

Toastify({
        progressBar: true,
        progressBarColor: 'coral',
        text: "With progress bar",
        gravity: "top",
        position: 'left'
    }).showToast();

Animation is done with CSS animation property.

[bengabp]

Please review and merge the pull requests, There are some features I need to use in my web project.

[DieterHolvoet]

If #112 gets merged, we'll have to make sure to restart the progress bar animation after re-focusing the page. I already wrote the code, but one of two PR's will have to be merged before we can add that.