refactor(cli): replace positional input argument with --input / -i flag

src/timesheets/cli.py

@@ -16,9 +16,10 @@ def build_parser() -> argparse.ArgumentParser:
 
     source = parser.add_mutually_exclusive_group(required=True)
     source.add_argument(
-        "input",
-        nargs="?",
+        "--input",
+        "-i",
         help="Path to the markdown file containing the timesheet table, or '-' to read from stdin.",
+        default=None,
     )
     source.add_argument(
         "--joplin",