Remove global config

This builds off the awesome config refactor to get us to a place where
most config is per-pry session.

Main changes:

1. Move from Pry::Pager to _pry_.pager
2. Use a Pry::Output to strip color when _pry_.config.color is false
3. Use Pry::Editor.new(_pry_) in place of Pry::Editor
4. Misc changes to stop referencing Pry.config in so many places