multiple build targets, pipeline

[deitch]

Fixes #12

• Builds for all combinations of [windows, linux, darwin] * [amd64, arm64].
• Creates makefile targets for build (local platform), build-all (all platforms), vet, test.
• Creates GitHub Actions CI pipeline for pull requests and merges, where it tests, vets, lints and builds.
• Creates GitHub Actions CI pipeline for tags with v* where it builds all of the binaries and cuts a release