Enable go modules

[radeksimko]

Go modules are becoming a de-facto standard in the Go community and adding go.mod files with metadata helps projects which may consume this repository as a dependency in deciding how to pin transitive dependencies and how to resolve conflicts.

This is a result of the following commands:

go mod init
go get ./...
go mod tidy

in a clean Go 1.11.5 environment.

[michaelklishin]

Thank you!