feat: Implement CLI and TUI for Fountain Pen Tracker

- Added CLI functionality for adding, editing, viewing, and deleting fountain pens.
- Introduced TUI using Textual for a more interactive experience.
- Created Pen and PenTracker classes to manage pen data and CSV storage.
- Implemented input validation for date fields.
- Added export functionality to JSON format.
- Updated project version to 0.2.0.
- Added unit tests for PenTracker functionality.

tests/test_engine.py

@@ -0,0 +1,37 @@
+import unittest
+import tempfile
+import os
+from pen_tracker.engine import PenTracker, Pen
+
+class TestPenTracker(unittest.TestCase):
+    def setUp(self):
+        self.temp_file = tempfile.NamedTemporaryFile(delete=False, suffix='.csv')
+        self.temp_file.close()
+        self.tracker = PenTracker(self.temp_file.name)
+
+    def tearDown(self):
+        os.unlink(self.temp_file.name)
+
+    def test_add_and_load_pen(self):
+        pen = Pen(Make="Pilot", Model="Metropolitan", Nib="F")
+        self.tracker.pens.append(pen)
+        self.tracker.save_data()
+        
+        # Load in new tracker
+        new_tracker = PenTracker(self.temp_file.name)
+        self.assertEqual(len(new_tracker.pens), 1)
+        self.assertEqual(new_tracker.pens[0].Make, "Pilot")
+        self.assertEqual(new_tracker.pens[0].Model, "Metropolitan")
+
+    def test_sorting(self):
+        self.tracker.pens = [
+            Pen(Make="Z", Model="A"),
+            Pen(Make="A", Model="B")
+        ]
+        self.tracker.save_data()
+        new_tracker = PenTracker(self.temp_file.name)
+        self.assertEqual(new_tracker.pens[0].Make, "A")
+        self.assertEqual(new_tracker.pens[1].Make, "Z")
+
+if __name__ == '__main__':
+    unittest.main()