Allow setting benthos cli flags from environment variables

[mikebaum]

When running benthos, it would be nice if it was possible to configure the CLI flags via environment variables. This is especially useful when running benthos in a docker container. Here are suggestions for the environment variables corresponding to the CLI flags:

• env-file - BENTHOS_ENV_FILE
• log.level - BENTHOS_LOG_LEVEL
• config - BENTHOS_CONFIG_FILE
• resources - BENTHOS_RESOURCE_FILES (comma separated list of files)
• chilled - BENTHOS_IGNORE_LINTING_ERRORS or BENTHOS_IGNORE_LINTER

Experimental Features:

• templates - BENTHOS_TEMPLATES (comma separated list of files)
• watcher - BENTHOS_WATCH_CONFIG

I would be more than willing to submit a PR for this.