Feature for selected Environments vars

[renato-farias]

This PR intends to add a new feature for selecting which env vars the caller wants to lookup and replace while using the cli or programmatically.

I believe it addresses what was requested in https://github.com/a8m/envsubst/issues/25.

Summary

• A new -envs added for listing what env vars we want to lookup and replace.

  echo 'hi ${USER} at ${PWD}' |envsubst -envs ENV,USER
  > hi myuser at ${PWD}

• Add new funcs for selecting envs programmatically:
  • StringSelectedEnvs envsubst.StringSelectedEnvs(input, []string{"ENV1"})
  • StringRestrictedSelectedEnvs envsubst.StringRestrictedSelectedEnvs(input, true, false, []string{"ENV1"})
  • BytesSelectedEnvs envsubst.BytesSelectedEnvs([]byte(input), []string{"ENV1"})
  • BytesRestrictedSelectedEnvs envsubst.BytesRestrictedSelectedEnvs([]byte(input), false, true, []string{"ENV1"})
  • ReadFileSelectedEnvs envsubst.ReadFileSelectedEnvs("filename", []string{"ENV1"})
  • ReadFileRestrictedSelectedEnvs envsubst.ReadFileRestrictedSelectedEnvs("filename", true, true, []string{"ENV1"})
• Add more tests