Splices a string, much like an array.
@param {String} s The string to operate on.
@param {Number} index The index to begin splicing the string at
@param {Number} count The number of characters to delete
@param {String} add The string to append at the spliced section
@return {String} The spliced string.
@function
function splice (s, index, count, add) {
                    return s.slice(0, index) + (add || '') + s.slice(index + count);
                }