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.

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:"