Batch fetching page texts

[RheingoldRiver]

Hi, I implemented this in my wrapper library a couple days ago: RheingoldRiver/mwcleric@7d0ee4a There's no support for anything like this in mwclient right now, right? Is this something that could be added?

I'd ideally like to adapt this and add it to site.allpages maybe if you supply a batch_size parameter and generator is True, but it could also be a separate method similar to what I've done - but I think something like this belongs in mwclient itself rather than a wrapper (see also #55). (Though maybe I'm just missing where this is already supported and could have saved the time to write this if I asked first lol)

Anyway assuming it's not currently supported and you would like it added, I'm happy to adapt my implementation, how would you like it done? Or if you would like, you can feel free to adapt my code as well if you want!

(also could you merge #264 btw? I've been needing to use move_subpages a bunch, I think that would be useful to have in the library)