Broken support for special prefixes

https://ruby-doc.org/stdlib-2.5.1/libdoc/readline/rdoc/Readline.html#method-c-special_prefixes  
  
I think this snippet gets us someway there but `@rl_special_prefixes` seems to be broken.  

```
module Readline
  def self.special_prefixes()
    @rl_special_prefixes
  end

  def self.special_prefixes=(prefixes)
    @rl_special_prefixes = prefixes
  end
end
```