Add support for passing maxlag to the API

[addshore]

Someone is currently using this code to make requests to the www.wikidata.org api.
They are not currently passing a maxlag value to the api (which they should).
I had a quick skim and it looks like support needs to be added.

I also have a ticket filed on the WMF phabricator instance tracking this
https://phabricator.wikimedia.org/T214064

[Xymph]

Thanks for the heads-up. Documentation: https://www.mediawiki.org/wiki/Manual:Maxlag_parameter

If someone submits a PR to implement this, I may be able to review it soon, but I won't have time to develop it myself for quite a while.

[Xymph]

This is now implemented in PR #112. @addshore, if you're able to shed some reviewing light on it, that would be welcome.

[Xymph]

@addshore This is released in v0.14.0, perhaps you can note it on Phabricator.