Make collapse transition duration dynamic

Created by: artifex404

Collapse.js transition duration is taken from a constant of value of 600.

However in some cases the transition needs to be altered with CSS, for example:

.collapsing {
    transition: none;
}

This prevents the collapsing trigger to work properly as it always waits for the 600ms timeout.

The collapse.js transition duration should take its value dynamically from css.

Demo: https://codepen.io/artifex404/pen/LewydR (Note: auto updating is disabled, rerun codepen to see changes)