allow stdin on rich.markdown.__main__

If user provides no PATH or uses "-" as PATH, markdown is read from sys.stdin.

Example:
python -m rich.markdown < README.md
cat README.md | python -m rich.markdown

CHANGELOG.md

@@ -5,6 +5,10 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [9.8.0]
+
+- Allow passing markdown over STDIN when using `python -m rich.markdown`
+
 ## [9.7.0] - 2021-01-09
 
 ### Added

CONTRIBUTORS.md

@@ -6,5 +6,6 @@ The following people have contributed to the development of Rich:
 
 - [Oleksis Fraga](https://github.com/oleksis)
 - [Hedy Li](https://github.com/hedythedev)
+- [Alexander Mancevice](https://github.com/amancevice)
 - [Will McGugan](https://github.com/willmcgugan)
 - [Nathan Page](https://github.com/nathanrpage97)

rich/markdown.py

@@ -527,11 +527,17 @@ class Markdown(JupyterMixin):
 if __name__ == "__main__":  # pragma: no cover
 
     import argparse
+    import sys
 
     parser = argparse.ArgumentParser(
         description="Render Markdown to the console with Rich"
     )
-    parser.add_argument("path", metavar="PATH", help="path to markdown file")
+    parser.add_argument(
+        "path",
+        metavar="PATH",
+        nargs="?",
+        help="path to markdown file",
+    )
     parser.add_argument(
         "-c",
         "--force-color",
@@ -587,14 +593,18 @@ if __name__ == "__main__":  # pragma: no cover
 
     from rich.console import Console
 
-    with open(args.path, "rt", encoding="utf-8") as markdown_file:
+    if not args.path or args.path == "-":
-        markdown = Markdown(
+        markdown_body = sys.stdin.read()
-            markdown_file.read(),
+    else:
-            justify="full" if args.justify else "left",
+        with open(args.path, "rt", encoding="utf-8") as markdown_file:
-            code_theme=args.code_theme,
+            markdown_body = markdown_file.read()
-            hyperlinks=args.hyperlinks,
+    markdown = Markdown(
-            inline_code_lexer=args.inline_code_lexer,
+        markdown_body,
-        )
+        justify="full" if args.justify else "left",
+        code_theme=args.code_theme,
+        hyperlinks=args.hyperlinks,
+        inline_code_lexer=args.inline_code_lexer,
+    )
     if args.page:
         import pydoc
         import io