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).
This seems risky.  Any site which itself checks this attribute could get quite confused.