Prompt from file proof of concept using plain text (#127)

This is a simple proof of concept of using external files as templates. 
I'm still feeling my way around the codebase.
As a user, I want to use files as prompts, so it will be easier to
manage and test prompts.
The future direction is to use a template engine, most likely Mako.

langchain/prompts/prompt.py

@@ -97,3 +97,18 @@ class Prompt(BaseModel, BasePrompt):
         example_str = example_separator.join(examples)
         template = prefix + example_str + suffix
         return cls(input_variables=input_variables, template=template)
+
+    @classmethod
+    def from_file(cls, template_file: str, input_variables: List[str]) -> "Prompt":
+        """Load a prompt from a file.
+
+        Args:
+            template_file: The path to the file containing the prompt template.
+            input_variables: A list of variable names the final prompt template
+                will expect.
+        Returns:
+            The prompt loaded from the file.
+        """
+        with open(template_file, "r") as f:
+            template = f.read()
+        return cls(input_variables=input_variables, template=template)

tests/unit_tests/data/prompt_file.txt

@@ -0,0 +1,2 @@
+Question: {question}
+Answer:

tests/unit_tests/test_prompt.py

@@ -77,3 +77,11 @@ def test_prompt_invalid_template_format() -> None:
         Prompt(
             input_variables=input_variables, template=template, template_format="bar"
         )
+
+
+def test_prompt_from_file() -> None:
+    """Test prompt can be successfully constructed from a file."""
+    template_file = "tests/unit_tests/data/prompt_file.txt"
+    input_variables = ["question"]
+    prompt = Prompt.from_file(template_file, input_variables)
+    assert prompt.template == "Question: {question}\nAnswer:"