Add support to backup pages using API:Query instead of Special:Export

[NyaMisty]

Since Special:Export page is using a huge bandwidth, some wiki sites have closed this page, making unable to dump without shell access. However they still accept API requests, so I hacked the script to achieve that. Hope helpful!
PS: using --apiexport can query pages through API.

[nemobis]

Thanks. Sorry for the long silence: this is important but only now I've had the occasion to test this in real life. We're especially concerned about keeping the XML as similar as possible to the traditional one and to the schema (which however may not be possible with this approach).

I've tried a streamlined allrevisions approach for MediaWiki 1.27+ wikis, which I've had in mind for a while, and then I added one based on the allpages/allrevisions API, like yours. #311

Now I'll compare the two approaches and see how to "merge" them.

[NyaMisty]

Thanks. I've only tested this script on two sites: zh.moegirl.org and hmoegirl.com.
And the generated format is consistent with the original script :)
But seems that the API interfaces of some wiki are not working correctly. Since you've tested your script in a greater scale of sites, I'll close my pr now.