Plugin signature (#520)

* calling plugin without shell

* spelling error

* option on register to select a shell

* help in plugin example signature

crates/nu_plugin_python/plugin.py

@@ -70,6 +70,14 @@ def signatures():
                     "var_id": None,
                 },
                 "named": [
+                    {
+                        "long": "help",
+                        "short": "h",
+                        "arg": None,
+                        "required": False,
+                        "desc": "Display this help message",
+                        "var_id": None
+                    },
                     {
                         "long": "flag",
                         "short": "f",