HTTP/2 support

[lud77]

I noticed this project is making use of Superagent version 3.7, which doesn't have support for http2.

Is there any plan to move on to the newest version of Superagent? They are currently at version 5.0.3 which provides http2 support through a .http2() method and since most browsers nowadays use http2 it would be very useful to be able to perform that kind of test.

[austince]

Certainly worth looking into upgrading superagent, though bumping to version 5.x would have to be a major release - we're currently looking into moving away from superagent (see #203) but I haven't had the time to finish that up yet. What kind of HTTP/2 support are you looking for? Can you provide some basic use/test cases?

[lud77]

We would basically like to be able to test that our publications are being correctly served over http2 in order to catch potential misconfigurations. I guess the basic use case would be to fetch a page from a website that should be serving over http2 and being able to verify that the page has actually been served using http2.