feat: Add /version endpoint and command line argument -version

[mloskot]

Is there any reason that go-httpbin binary does not seem to offer any way to report its version?

I there isn't any, I'd like to suggest adding it, if possible.

Additionally, it would be nice to have /version endpoint.

I often have to test number of environments, clusters, DNS zones/domains, where I deploy different versions of go-httpbin, and being able to request the release version would be useful as a way to identify certain deployments.

If there is an interest, I'm keen in giving it a shot to implement it.

[mccutchen]

No particular reason. Both additions sound like a good idea to me, I’ll gladly review a pull request if you open one!

[mloskot]

Good to hear, I've proposed #191