Move to go modules

[mre]

I think it would be nice to move the project to Golang modules, that were introduced in 1.11.
Tried in other projects and it works quite well.
Example for different project: mre/riffraff#3

Pull requests very welcome. We can provide mentorship if needed. Just comment here.

[xenowits]

@mre can i take this up??

[mre]

Go for it @xenowits. 😃

[xenowits]

Thanks :)