createButton({content, className, tagName = 'button', attrs = {}}) {
        let _content = typeof content === 'function' ? content(this.dp) : content;

        return createElement({
            tagName,
            innerHtml: `<span tabindex='-1'>${_content}</span>`,
            className: classNames('air-datepicker-button', className),
            attrs
        });
    }
Creates datepicker button HTML element
@param {String|Function} content - button content
@param {String} [className]
@param {String} [tagName=button]
@param {Object} [attrs]
@return HTMLElement