Similar to

• Reading keys from environment variables cloudlist#249
• Support for reading keys from environment variables in report config file nuclei#3015

Please describe your feature request:

Allow reading reporting config values from as env variable to avoid storing sensitive keys/credentials as plain text in a reporting and flag config file.

$ cat config.yaml

slack:
  - id: recon
    slack_channel: $SLACK_CHANNEL
    slack_username: $SLACK_USER
    slack_webhook_url: $SLACK_WEBHOOK

Describe the use case of this feature:

cat output | notify -pc config.yaml