Make script optional+pass everything to subprocess

This restores previous behaviour of the pythonSoftIOC executable, which
would give you a Python command line interpreter.

Author: AlexanderWells-diamond

softioc/__main__.py

@@ -9,12 +9,17 @@
 def main(args=None):
     parser = ArgumentParser()
     parser.add_argument("--version", action="version", version=__version__)
-    parser.add_argument("script", help="The python script to run")
+    parser.add_argument(
+        "script", help="The python script to run", nargs="?", default=None)
     parser.add_argument(
         "arg", help="Any arguments to pass to the script", nargs="*")
-    parsed_args = parser.parse_args(args)
-    # Execute as subprocess
-    cmd = [sys.executable, parsed_args.script] + parsed_args.arg
+    parsed_args, unknown = parser.parse_known_args(args)
+
+    # Execute as subprocess.
+    cmd = [sys.executable] + parsed_args.arg + unknown
+    if parsed_args.script:
+        cmd.insert(1, parsed_args.script)
+
     subprocess.Popen(cmd).communicate()