Add profile with HTTP/2 support

[Juholei]

As far as I have understood, none of the web server options come with HTTP/2 support configured out of the box. I think it would be nice and useful to have an option where HTTP/2 would be working.

[yogthos]

Makes sense, it looks like for Immutant it's just a matter of setting the flag here.

[yogthos]

Is there any reason not to set that as the default?

[Juholei]

I'm not that familiar with the matter, but at least in practice enabling HTTP/2 requires also enabling SSL, so maybe that could be one reason not to set it as default.

[yogthos]

Yeah that would make sense, so I guess that would need to be a profile that sets up SSL as well. That gets a bit trickier because you actually need to have certs. It might be worth experimenting with Immutant since it's just a case of adding a key in the initialization options.

[coreagile]

I have some code for generating a self-signed cert for Jetty built on top of luminus if that would help. It's not the cleanest implementation, but might provide a jumping-off point? Oh, and I use HTTP/2 as well using [info.sunng/ring-jetty9-adapter "0.18.3"] but I have no idea if it's any good.