ref: strict argument matching for `Plugin.command` deco

Previously any keyword arguments passed into the `Plugin.command`
decorator that didn't match a keyword argument inside of `Comman.update`
would be collected into the `metdata` for a command.

This can cause fairly severe bugs in your code if you make typos when
passing keyword arguments into the decorator since they will be blindly
accepted. For example consider a situation where a user (lets say me)
creates the following command:

```
@Plugin.command(..., permissions=CommandLevels.TRUSTED)
```

While this looks perfectly valid and the code will run without issue,
this command actually has a permissions level of '0' since the correct
keyword argument is `level`.

This commit changes the behavior to require all metadata passed in to be
within a `metadata` kwarg. Any unknown keyword arguments passed will
throw a traditional Python error.

disco/bot/command.py

@@ -171,7 +171,7 @@ class Command(object):
             oob=False,
             context=None,
             parser=False,
-            **kwargs):
+            metadata=None):
         self.triggers += aliases or []
 
         def resolve_role(ctx, rid):
@@ -209,7 +209,7 @@ class Command(object):
         self.is_regex = is_regex
         self.oob = oob
         self.context = context or {}
-        self.metadata = kwargs
+        self.metadata = metadata or {}
 
         if parser:
             self.parser = PluginArgumentParser(prog=self.name, add_help=False)