add auto completion for _pry_.config.so<tab>.

[ghost]

_pry_.config dispatches almost every method call through method_missing. only three methods are defined directly on Pry::Config: #pry, #output=, and #hooks=. we might add more later for the really common method calls like #input and #output.

the problem this introduces is that almost no configuration options can be auto-completed on _pry_.config and that's sad.

[ConradIrwin]

Duplicate of #1277