Improve helpers

[moul]

• [x] Isolate helpers in their own package
• [ ] Export some functions
• [ ] Setup godoc.org badge
• [ ] Deprecate the deprecated helpers
• [ ] Unit test helpers (with examples)
• [ ] Prefix helpers that are language-specific

[moul]

Godoc link is here: https://godoc.org/github.com/moul/protoc-gen-gotemplate/helpers, but not yet interesting, it needs exported functions or go test examples