Imgur: use smaller thumbnails via srcset attribute
Imgur: use smaller thumbnails via srcset attribute

- take advantage of 160,320,640 and 1024px versions
- use regular `<img>` with `srcset` attribute
- closes #124

lidel/yt-looper,lidel/yt-looper,sk4zuzu/yt-looper,lidel/yt-looper,sk4zuzu/yt-looper,sk4zuzu/yt-looper