Custom variable substitutions from external file

[esune]

Rather than replacing the default file containing the variable substitution definitions/implementations, it would be better to allow users to:

• specify a path to a python file containing custom definitions (or overrides) for the variable substitutions
• if set, try to load the new definitions and add them to the map of existing substitutions
  • nice to have: log which custom substitutions were loaded
• if set, but file does not exist/is unreachable, log a warning and carry on