unit test / code coverage improvements (#322)

This PR has two contributions:

1. Add test for when stop token is found in middle of text

2. Add code coverage tooling and instructions
- Add pytest-cov via poetry
- Add necessary config files
- Add new make instruction for `coverage`
- Update README with coverage guidance
- Update minor README formatting/spelling

Co-authored-by: Hunter Gerlach <hunter@huntergerlach.com>

tests/unit_tests/llms/test_utils.py

@@ -10,6 +10,9 @@ def test_enforce_stop_tokens() -> None:
     text = "foo bar baz"
     output = enforce_stop_tokens(text, ["moo", "baz", "bar"])
     assert output == "foo "
+    text = "foo bar baz"
+    output = enforce_stop_tokens(text, ["moo", "bar"])
+    assert output == "foo "
 
 
 def test_enforce_stop_tokens_none() -> None: