Add httpie --version (#1220)

httpie/manager/cli.py

@@ -1,5 +1,6 @@
 from textwrap import dedent
 from httpie.cli.argparser import HTTPieManagerArgumentParser
+from httpie import __version__
 
 COMMANDS = {
     'plugins': {
@@ -90,4 +91,14 @@ parser.add_argument(
     '''
 )
 
+parser.add_argument(
+    '--version',
+    action='version',
+    version=__version__,
+    help='''
+    Show version and exit.
+
+    '''
+)
+
 generate_subparsers(parser, parser, COMMANDS)