…s of all types

Some config parameters, like 'exit on error', 'async' and 'working directory', are generally applicable to all/most cogs.
It makes sense to have a consistent definition for their setter/getter methods.

Many of these definitions exist on the Cog::Config base class, but user still need to specify them multiple times for each cog type.

Now you can set some options globally using a single block

```
config do
  global do
    # every cog of every time will run with this attribute set, unless overridden by a more specific config specification
    exit_on_error!
  end

  cmd do
    # overrides for all `cmd` cogs
    no_exit_on_error!
  end

  chat(:foo) do
    # overrides for a specifically named `chat` cog
    no_exit_on_error!
  end
end
```