request: quiet mode enhancements

[mcandre]

• Assume quiet mode by default, following UNIX console logging conventions. For example, accept a CLI flag --verbose to opt into information logs.
• Accept an environment variable JINJANATOR_QUIET (default blank). When present, enable quiet mode. This deduplicates the --quiet flag when calling jinjanator in many separate shell commands.

[kpfleming]

The tool already follows UNIX console conventions by sending the version information (and errors) to stderr, not stdout. Any script which uses the output generated by the tool will read from stdout and not be affected by it. I would consider a PR to change the code to only print version information when --version is specified, though, as it doesn't need to be output on every run.