Allow PostUp and PostDown commands #52
Description
As the title describes, it would be more flexible if nylon allows users to run commands when a service/interface starts or stops.
This is useful if we want to make use cases such as #41 more user-friendly.
Along with this, we could also make the nylon interface more flexible, i.e allow different services to have different interfaces. However, this may also add complexity.
Security Considerations:
a) We should only allow PostUp and PostDown commands to be configured locally in node.yaml, it's probably unwise to allow this to be configured centrally (RCE, cross-platform).
b) We need to ensure file permissions are set correctly. On startup, we should check if the file is readable/writable by others, and throw a warning or error accordingly.