fix the failing Pry#prompt specs.

this is a strange and odd case. Pry.prompt is a delegate to Pry.config, as
it has always been. the same delegate was setup on an instance of Pry, but
never used because we define #prompt and #prompt= with our implementation.

the thing that would make the most sense (to me) is to not support Pry.prompt
anymore and recommend the use of Pry.config.prompt instead. a lot of code relies
on Pry.prompt though, so we have to support the delegate to config and implement
custom behavior for the pry instance.

lib/pry/config/convenience.rb

@@ -3,7 +3,6 @@ module Pry::Config::Convenience
                :input,
                :output,
                :commands,
-               :prompt,
                :print,
                :exception_handler,
                :quiet?,

lib/pry/pry_class.rb

@@ -20,6 +20,14 @@ class Pry
 
     extend Pry::Config::Convenience
     config_shortcut *Pry::Config.shortcuts
+
+    def prompt=(value)
+      config.prompt = value
+    end
+
+    def prompt
+      config.prompt
+    end
   end
 
   def self.main

lib/pry/pry_instance.rb

@@ -69,6 +69,7 @@ class Pry
     @backtrace     = options[:backtrace] || caller
     @config = Pry::Config.new
     @config.merge!(options)
+    push_prompt(config.prompt)
     @input_array  = Pry::HistoryArray.new config.memory_size
     @output_array = Pry::HistoryArray.new config.memory_size
     @custom_completions = config.command_completer
@@ -78,6 +79,26 @@ class Pry
     exec_hook(:when_started, options[:target], options, self)
   end
 
+  # The current prompt.
+  # This is the prompt at the top of the prompt stack.
+  #
+  # @example
+  #    self.prompt = Pry::SIMPLE_PROMPT
+  #    self.prompt # => Pry::SIMPLE_PROMPT
+  #
+  # @return [Array<Proc>] Current prompt.
+  def prompt
+    prompt_stack.last
+  end
+
+  def prompt=(new_prompt)
+    if prompt_stack.empty?
+      push_prompt new_prompt
+    else
+      prompt_stack[-1] = new_prompt
+    end
+  end
+
   # Initialize this instance by pushing its initial context into the binding
   # stack. If no target is given, start at the top level.
   def push_initial_binding(target=nil)