Don't depend on markup for lazy load the images

The lazy load strategy was depending on user's markup. The `src` tag could
not be present in the HTML, because Slick would understand it as loaded. Now,
it depends solely on `data-lazy` attribute: its presence will force the load,
and its absence will indicate it is already loaded.
Progressive lazy load now also do not depend on markup
Merge remote-tracking branch 'bellthoven/lazy-load-markup-independant' into lazy-load-func

* bellthoven/lazy-load-markup-independant:
  Progressive lazy load now also do not depend on markup
  Don't depend on markup for lazy load the images
Add an onLoadImages function

Allows customizing how images are lazy loaded. Left the default
'img[data-lazy]' way untouched.

Includes an option 'onLoadImages' which defaults to null.
Fix rangeStart and rangeEnd when in centerMode

Uses half the count of slidesToShow to find the start and end.
Merge branch 'center-mode-lazy-load-fix'

* center-mode-lazy-load-fix:
  Fix rangeStart and rangeEnd when in centerMode

Conflicts:
	slick/slick.js