Renames loglines*.py -> strings*.py (exercism#1291)

The existing naming would conflict with the expectations of the
test runner / representer / analyzer tooling.

Author: yawpitch

exercises/concept/strings/loglines.py renamed to exercises/concept/strings/strings.py

@@ -1,5 +1,5 @@
 import unittest
-from loglines import extract_message, change_log_level, reformat
+from strings import extract_message, change_log_level, reformat
 
 
 class TestLogLines(unittest.TestCase):