Drone plugin for Helm 3

TODO:

• Make a .drone.yml that's sufficient for building an image
• Make a Dockerfile that's sufficient for launching the built image
• Make cmd/drone-helm/main.go actually invoke helm
• Make golint part of the build process (and make it pass)
• Implement debug output
• Flesh out helm upgrade until it's capable of working
• Implement config settings for upgrade
• Implement helm lint
• Implement helm delete
• Look for command-line flags added in helm3; implement them
• EKS support
• Dotenv support
• Example drone config in this README
• Change .drone.yml to use a real docker registry

Nice-to-haves:

• Cleanup() method on Steps to close open filehandles, etc.
• Replace fmt.Printf with an actual logger
• Replace fmt.Errorf with github.com/pkg/errors.Wrap, since the built-in Unwrap doesn't work the way Cause does
• Deprecation warnings if there are environment variables that aren't applicable in helm3