create main routine with argparsing

src/vigenere/vig.py

@@ -0,0 +1,11 @@
+if __name__ == "__main__":
+    import argparse
+    parser = argparse.ArgumentParser()
+    parser.add_argument('FILE')
+    command_group = parser.add_mutually_exclusive_group(required=True)
+    command_group.add_argument('--encrypt', metavar='KEY')
+    command_group.add_argument('--decrypt', metavar='KEY')
+    parser.add_argument('--out', metavar='FILE')
+
+    args = parser.parse_args()
+