initialize Pry.config in Pry.reset_defaults(revisit later).

we don't want to depend on Pry.reset_defaults if we can avoid it.
`Pry.config` being a lazy-loaded value has benefits. I'll try to
revisit this later when the rest of the new work on Pry::Config
and Pry::Config::Default is finished.
stringify keys passed to Pry::Config#[] and Pry::Config#[]=.