Please show me how will you write that without jQuery please :) (It's not I'm not able to do it, but you already know that right ?)

toggle.js

		// togglable ability (used for navbar)
		$('.js-togglable').each(function() {
			var togglableContainer = this;
			$('.js-togglable__toggler', togglableContainer).on('click keyup', function() {
				$('.js-togglable__item', togglableContainer).toggleClass('js-togglable__item--hide');
			});
		});

So I just need to add
NodeList.prototype.forEach = Array.prototype.forEach; to make this work like a charm. Bye jQuery :)
Thanks @anthonyshort !
Now I just need a nice way to includes polyfills to support old browsers :)

I found an other way for the forEach trick.

Array.prototype.slice.call(document.querySelectorAll('.js-togglable')).forEach(function(el){
    var toggler = el.querySelector('.js-togglable__toggler');
    var items = Array.prototype.slice.call(el.querySelectorAll('.js-togglable__item'));

    var toggle = function(event) {
        items.forEach(function(item) {
            item.classList.toggle('js-togglable__item--hide');
        });
    };

    toggler.addEventListener('click', toggle);
    toggler.addEventListener('keyup', toggle);
});