Proposal: Prepare v2

[smola]

There is a few things that are pending and may be improved in a v2 version. It would make sense to start thinking about it once Go 1.12 is out, specially if we consider to adopt Go modules and phase out go dep.

This would be my wishlist, it's just something to get started:

• Add support for Go modules, remove go dep stuff. This is still something that needs discussion, since we'll still keep using go dep in the short term.
• Enable windows builds by default. Make it explicit when disabling them instead.
• Specify PKG_PLATFORMS instead of PKG_OS and PKG_ARCH, so we have finer-grained control over GOOS/GOARCH pairs that are built.
• Drop support for older Go versions (e.g. Go 1.11 if we adopt modules).
• Simplify test-coverage by adopting the new support for coverage on multiple packages (this depends on dropping support for Go <=1.9) Go: use coverprofile with multiple packages when upgrading to Go 1.10 #64
• GitHub Actions support