Commands: add option documentation in docstring.

This allows plugins to document the commands they add.

The docstring is parsed as a regular expression:

    arg:<type>:<name>:<description>\n

Types are defined in commands.arg_types.

Note that this commit removes support for single letter
shortcuts in command options.

If a command is not properly documented, a warning is issued
with print(), because no logger is available at this point.

electrum/plugins/labels/__init__.py

@@ -15,5 +15,9 @@ async def push(self: 'Commands', plugin: 'LabelsPlugin' = None, wallet=None) ->
 
 @plugin_command('w', plugin_name)
 async def pull(self: 'Commands', plugin: 'LabelsPlugin' = None, wallet=None, force=False) -> int:
-    """ pull labels from server """
+    """
+    pull missing labels from server
+
+    arg:bool:force:pull all labels
+    """
     return await plugin.pull_thread(wallet, force=force)