Add option to disable auto sync

[andreyorst]

Hello!

I have a limited amount of cellular traffic on my phone so I'd like to avoid an automatic feed update. I have a lot of feeds, many of which often include large images and it can drown my traffic pretty quickly. I've outlined my use-case in #734, so this is also kind of related to being able to keep the app offline for reading. I don't want to only fetch articles when I'm on wifi, sometimes I do want to fetch when I'm on cellular, while I still have lots of traffic (it updates monthly), so the ability to manually fetch articles would solve this problem.