[commands] Allow converters to be instantiated.

This allows for you to create converters that can have varying
behaviour using the converter's __init__ instead of having to do a
meta-class based approach to get around the fact that __init__ is part
of the interface.

To make up for the lack of __init__, a new method Converter.prepare was
added to do the work that __init__ used to do.

discord/ext/commands/core.py

@@ -203,11 +203,15 @@ class Command:
             converter = getattr(converters, converter.__name__ + 'Converter')
 
         if inspect.isclass(converter) and issubclass(converter, converters.Converter):
-            instance = converter(ctx, argument)
-            if asyncio.iscoroutinefunction(instance.convert):
-                return (yield from instance.convert())
-            else:
-                return instance.convert()
+            instance = converter()
+            instance.prepare(ctx, argument)
+            ret = yield from discord.utils.maybe_coroutine(instance.convert)
+            return ret
+
+        if isinstance(converter, converters.Converter):
+            converter.prepare(ctx, argument)
+            ret = yield from discord.utils.maybe_coroutine(converter.convert)
+            return ret
 
         return converter(argument)