[commands] custom default arguments

Modeled slightly after Converters, allow specifying Converter-like class
for context-based default parameters. e.g.

```py
class Author(ParamDefault):
  async def default(self, ctx):
    return ctx.author

async def my_command(ctx, user: discord.Member=Author):
  ...
```

Also adds a few common cases (Author, Channel, Guild) for current
author, ...

discord/ext/commands/core.py

@@ -419,7 +419,7 @@ async def _resolve_default(self, ctx, param):
             if inspect.isclass(param.default) and issubclass(param.default, defaults.CustomDefault):
                 instance = param.default()
                 return await instance.default(ctx=ctx, param=param)
-            elif isinstance(param.default, converters.CustomDefault):
+            elif isinstance(param.default, defaults.CustomDefault):
                 return await param.default.default(ctx=ctx, param=param)
         except CommandError as e:
             raise e