Allow customisation of the prompt objects

Fix issue #885 (Add an API for extending Pry.view_clip)

    [1] pry(main)> class Barbie
                 |   def inspect
                 |     'You can brush my hair, undress me everywhere!'
                 |   end
                 | end
    => nil
    [2] pry(main)>
    [3] pry(main)> Pry.config.prompt_safe_objects << Barbie
    => [String, Numeric, Symbol, nil, true, false, Barbie]
    [4] pry(main)> cd Barbie.new
    [5] pry(You can brush my hair, undress me everywhere!):1>