Allow opening/zooming preview image

[SpitFire-666]

Basically, when tapping on the preview image in the feed, RedReader goes blank and loads the original, hi res image. This takes time and a lot of the time I'm just wanting to make the already loaded preview bigger (pinch to zoom).

Most of the time, the preview is of good enough quality for me, and I don't need to download multi megapixel pics on my handheld device 😀