Allow preloading of all thumbnails to be disabled

[bikegriffith]

With a very long movie, preloading all of the thumbnails becomes network intensive. Allowing them to load on-demand (and fill the cache on hover) should be an option.

[tjenkinson]

How many thumbnails are you loading? For my case the number of thumbnails doesn't really depend on the length of the video, I have configured a maximum number to generate.
Thanks.

[bikegriffith]

@tjenkinson I generate one for every 10 seconds of video (so 4 hour video --> 1440 thumbs). Good call on maximum number though; I can easily slice up my list on the client side before passing into Clappr so that you can only scrub through a new thumbnail every x pixels.

[tjenkinson]

Cool ok. A new option to only load on first hover might be useful in case the user never hovers over the bar, so I'll leave this issue open.