Preload article + enhance autofocus

Better usability:
* Added preload option, to fetch articles in background, before they are opened
 - Next/previous item are really fast
* Enhance autofocus option
 - Scroll if needed (article not fully visible)

Note about preload option:
Sucks in case there is auto-play audio/video...
TODO: store data in cache, and fill the DOM only when switching item?
[this will require some code refactoring and changes, maybe in another commit...]

Also:
- jshint -> fix errors
- fix favicon
- fix js error on start