lazy load readline through Proc defined on Pry::Config::Default.

I think we might try to reference 'Readline' in the class body of a few classes. I'll try to look into that as well. we should probably move away from the assumption that 'readline' is always being used with pry. maybe it is `pry-coolline` and the require of 'readline' is not needed.

closes #1107
closes #1081
closes #1095