Error when overriding default prompt template of ConversationChain 
yes you're correct - conversation chain only currently allows for a single input, that being the input key, and then also history (coming from memory) How should I handle this situation if I want to add _context_?

```
system_template="""Use the following pieces of context to answer the users question. 
If you don't know the answer, just say that you don't know, don't try to make up an answer.
----------------
{context}"""
messages = [
    SystemMessagePromptTemplate.from_template(system_template),
    MessagesPlaceholder(variable_name="history"),
    HumanMessagePromptTemplate.from_template("{input}")
]
```
Hi, does anyone know how to override the prompt template of ConversationChain? I am creating a custom prompt template that takes in an additional input variable 
```
PROMPT_TEMPLATE = """ {my_info}
{history}
Human: {input}
AI:"""

PROMPT = PromptTemplate(
    input_variables=["history", "input", "my_info"], template=PROMPT_TEMPLATE
)

conversation_chain = ConversationChain(
    prompt=PROMPT,
    llm=OpenAI(temperature=0.7), 
    verbose=True, 
    memory=ConversationBufferMemory()
)
```

but got the following error:
```
Got unexpected prompt input variables. The prompt expects ['history', 'input', 'my_info'], but got ['history'] as inputs from memory, and input as the normal input key. (type=value_error)
```

Is my understanding correct that currently ConversationChain can only support prompt template that takes in "history" and "input" as the input variables?