feat: add support for calling specific functions for python prompt

[typpo]

Related to #507

Example:

promptfooconfig.yaml:

prompts:
  - prompt_python.py:prompt1
  - prompt_python.py:prompt2
providers: 
  - openai:gpt-3.5-turbo-0613
tests:
  - vars:
      topic: bananas

prompt_python.py:

def prompt1(context):
    return f'Write a one-sentence compliment for anyone who likes {context["vars"]["topic"]}.'

def prompt2(context):
    return f'Describe {context["vars"]["topic"]} concisely, comparing it to the Python programming language.'