feat: add version option to markitdown CLI (#172)

Add a `--version` option to the markitdown command-line interface 
that displays the current version number.

src/markitdown/__main__.py

@@ -1,15 +1,17 @@
 # SPDX-FileCopyrightText: 2024-present Adam Fourney <adamfo@microsoft.com>
 #
 # SPDX-License-Identifier: MIT
-import sys
 import argparse
+import sys
 from textwrap import dedent
+from .__about__ import __version__
 from ._markitdown import MarkItDown, DocumentConverterResult
 
 
 def main():
     parser = argparse.ArgumentParser(
         description="Convert various file formats to markdown.",
+        prog="markitdown",
         formatter_class=argparse.RawDescriptionHelpFormatter,
         usage=dedent(
             """
@@ -41,6 +43,14 @@ def main():
         ).strip(),
     )
 
+    parser.add_argument(
+        "-v",
+        "--version",
+        action="version",
+        version=f"%(prog)s {__version__}",
+        help="show the version number and exit",
+    )
+
     parser.add_argument("filename", nargs="?")
     parser.add_argument(
         "-o",