Add go.mod

[mitchellh]

This adds a go.mod file, automatically made using:

$ go mod init
$ go test

This doesn't add any versioning yet to this module, we can tag it v1 after if you're comfortable, but I want to double check deps first.

[evanphx]

100% of those dependencies are test dependencies. Unsure how go.mod handles that (or if it should), as someone using go-hclog doesn't really need those things unless they're developing it.

[evanphx]

Answered my own question: golang/go#26913 (comment)

[mitchellh]

Yeah! If we need to lower the minimum version in this go.mod for downstreams we can do that after, but given the dependencies listed I think it'll be fine.