Enable link hints for all elements with `click` listeners

This PR implements a hook on `addEventListener`, which marks elements registering a `click` event listener so that Vimium recognises them.

This enables link hint support for clickable elements such as eg. the quiz choices on [this Buzzfeed quiz](http://www.buzzfeed.com/adamellis/this-palm-reading-quiz-will-reveal-your-future#2s6on3d) or the slideshow navigation buttons on the [Mercedes website](http://www5.mercedes-benz.com/en/).

This PR

* fixes #1091 
* fixes #993 
* fixes #861 
* improves the situation for #1157 (some clickable elements still don't receive link hints).
I'll try and debug this. We really need to push on the chrome issue so we don't have to do this at all, it's ridiculous.