Add support for Go modules

[flimzy]

Now that Go 1.13 has been released, it seems Go modules aren't going to be changing substantially, so I think it's time to add Go mod support to Kivik.

That may make it reasonable/desirable to merge back into a monorepo, as well, but that needs to be further investigated before making a decision, and wouldn't be reasonable to do until a new major release (probably Kivik 3.0)

[flimzy]

This was resolved with the release of Kivik 3.