Update all existing plugins to work with conversations

[simonw]

Following:

• Introduce conversations concept for continue mode #85

TODO:

• Update for conversation support llm-gpt4all#4
• Update for conversation support llm-palm#3
• Update for conversation support llm-mpt30b#2
• Update for conversation support llm-markov#6

[simonw]

The execute() signature has changed to:

def execute(self, prompt, stream, response, conversation):

[simonw]

The hardest one is this:

• Update for conversation support llm-gpt4all#4

Because the various models supported by that plugin often have different ways of arranging their prompts.