Adding go.mod support. Closes #193

[Dragomir-Ivanov]

No description provided.

[rs]

We should not include test deps here.

[Dragomir-Ivanov]

Hmm, I have never worked with mod before, so I read some for this patch.
I find this explanation:

The module download is split into two parts: downloading the go.mod and downloading the actual code. If you have dependencies only needed for tests, then they will show up in your go.mod, and go get will download their go.mods, but it will not download their code. The test-only dependencies get downloaded only when you need it, such as the first time you run go test.

source: golang/go#26913 (comment)

Also, predictability of the testing is as important as the main project.

[smyrman]

We could potentially exclude example dependencies by adding a go.mod file to the examples directory and running go build and/or go mod tidy on the relevant packages.

[smyrman]

Maybe I can just do it :-)

[smyrman]

Moved to #210.