Make internal helper functions private

Added underscore prefix to functions used only internally:
- _strip_tags_and_truncate (used only in parse_post_file)
- _parse_headers (used only in parse_post_file)
- _rfc_2822_format (used only in parse_post_file)
- _make_links_absolute (used only in parse_post_file)
- _strip_html_tags (used only in parse_post_file)
- _get_friendly_date (used only in _setup_page_params)
- _exit_program (used only in monitor.py main loop)

This improves encapsulation by clearly marking implementation details.

monitor.py

@@ -28,7 +28,7 @@ def get_comment_count():
     return 0 if not resp.ok else int(resp.json()["count"])
 
 
-def exit_program():
+def _exit_program():
     """Exit the program with status code 0."""
     sys.exit(0)
 
@@ -52,7 +52,7 @@ while True:
     for _ in range(15):
         time.sleep(60)
         if initial_count != get_comment_count():
-            exit_program()
+            _exit_program()
     # check if git repo changed every 15 minutes
     if external_check_cmd and os.system(external_check_cmd):
-        exit_program()
+        _exit_program()