Add offline reading support

[xavez]

I know appcache is a bitch, but support for offline reading in browsers that support it would be great.

[monreal]

Would be great for mobile browsers.

[GLLM]

That would be just fantastic, given that I loose 45min twice a day and since Paris metro is not great for mobile reception ... no Selfoss reading & syncing when back outside !!

Maybe, as a starter, we could get all Selfoss "static" data stored : icons, png & co ?

Ideas :

• ability to set a feed as cacheable/storable, so that only wanted feeds gets stored, and not all of them (sometimes, those having the biggest numbers of items are not the prefered ones)
• ability to define the max number of items to store globally
• ability to define the max number of items to store per feed

Anyone has ideas ?

[niol]

I've worked on this and it is usable for me. Any feedback would be welcome. I'll make a pull-request when I've used it a bit more. https://github.com/niol/selfoss/tree/ppr/offline

[jtojnar]

This has been implemented in #1014