Don't zoom to uselessly small elements in zoom.js plugin [on demand]

[dainiak]

Often it's reasonable to zoom not to the innermost HTML node that is clicked but to some other (ancestor) node. This turns especially useful while working with MathJax or highlihgting plugins: they create many helper HTML elements, and usually clicking on them in reveal.js presentation is useless. I propose adding a CSS class zoomable, so that when an element is clicked, if it's contained in some "zoomable" element, the closest of them in the DOM hierarchy is being zoomed instead of initial mouseclick event target. If the're no such elements on the current slide, fall back to traditional behaviour. This can be achieved by altering a small code block in zoom.js plugin.