Enable addition of multiple markdown files

For the python cli, let specify multiple files rather than a single one.

README.md

@@ -42,7 +42,7 @@ Description of your event.
 Add the event using:
 
 ```bash
-python -m git_calendar --add [event.md]
+python -m git_calendar --add event1.md [event2.md ...]
 ```
 
 See help menu for more options:

git_calendar/__main__.py

@@ -6,9 +6,10 @@ def main():
     parser = argparse.ArgumentParser(description="Manage your git-based calendar.")
     parser.add_argument(
         "-a", "--add",
+        nargs="+",
         metavar="FILENAME",
         type=str,
-        help="Add an event from the specified markdown file."
+        help="Add events from the specified list of markdown files."
     )
     parser.add_argument(
         "--regenerate",
@@ -29,8 +30,9 @@ def main():
     calendar = GitCalendar(args.ics_file)
 
     if args.add:
-        print(f"Adding event from file: {args.add}")
-        calendar.add_event(args.add)
+        for event_mdfile in args.add:
+            print(f"Adding event from file: {event_mdfile}")
+            calendar.add_event(event_mdfile)
 
     if args.regenerate:
         print("Regenerating calendar file.")