gosubst v0.4.0

gosubst does it again!

Updates for v0.4.0 include:

• (BREAKING CHANGE for all 0 people who is using this tool) the removal of .Env from the context; realized that Sprig's env() did the job cleaner and better;
• adds the functions requiredEnvs, requiredVals and requiredFiles to templates: seems useful after debugging missing env vars, bad expansion, or attempts to {{ sh "cat nofile" }} for missing files;
• better tests.