Add environment variable support to pixy.yaml files

[cryptaliagy]

Using something like minijinja, Pixy could have support for pulling environment variables into values for the targets. This would be most useful for adding things like authentication mechanisms (basic auth/bearer auth) for webhooks, as well as for credential access for future targets

[cryptaliagy]

This will also unblock #1 #2 #3, since a big concern of mine was being able to implement these targets with better security than was available in the Enviro boards that have them statically define things like passwords in the config.py on the board