implement Pry global and pry-local configuration.

maybe best explained with an example:
      [1] pry(main)> Pry.config.hi = "hi"
      => "hi"
      [2] pry(main)> _pry_.config.hi
      => "hi"
      [3] pry(main)> _pry_.config.x = 1
      => 1
      [4] pry(main)> Pry.config.x
      => nil
      [5] pry(main)>

'Pry.config' is an instance of Pry::Config::Default. it defines the initial
default configuration values an instance of 'Pry' is going to have. an instance
of 'Pry' uses an instance of Pry::Config who relies on Pry.config as a default
or fallback for keys it cannot find locally.

for example 'Pry.config.color = true' is seen by _all_ pry REPLs who are active
but _pry_.config.color = false is seen only by the REPL session being interacted
with. a number of "config shortcircuts" are still available, for example it is
possible to say `_pry_.input = StringIO.new` or `Pry.input = StringIO.new`. the
shortcuts are maintained for the Pry class and instances of the Pry class through
Pry::Config.shortcuts.

Pry::Config::Convenience adds a method called 'config_shortcuts' which can be used
to setup shortcut access to 'some_obj.config.blah' as 'some_obj.blah'

a lot of tests still fail, so work in progress.

this should help solve #1055.