Add a --version command line flag to show the version

This will help users report the version of pystack when creating bug
reports more easily.

Signed-off-by: Pablo Galindo <pablogsal@gmail.com>

news/203.feature.rst

@@ -0,0 +1 @@
+Add a new ``--version`` command line flag to show the version of pystack

src/pystack/__main__.py

@@ -12,6 +12,7 @@
 from typing import Optional
 from typing import Set
 
+from pystack import __version__
 from pystack.errors import InvalidPythonProcess
 from pystack.process import decompress_gzip
 from pystack.process import is_elf
@@ -118,6 +119,9 @@ def generate_cli_parser() -> argparse.ArgumentParser:
     parser.add_argument(
         "-v", "--verbose", action="count", default=0, dest="global_verbose"
     )
+    parser.add_argument(
+        "--version", action="version", version=__version__, help="Show version"
+    )
     parser.add_argument(
         "--no-color",
         action="store_true",