Show the contents of the process method by default

When creating a command using a custom subclass there may not be a
readily available block. In that case defaulting to the process method
seems reasonable for now.

lib/pry/command.rb

@@ -14,7 +14,7 @@ class Pry
     # Properties of the command itself (as passed as arguments to
     # {CommandSet#command} or {CommandSet#create_command}).
     class << self
-      attr_accessor :block
+      attr_writer :block
       attr_writer :description
       attr_writer :command_options
       attr_writer :match
@@ -45,6 +45,10 @@ class Pry
         @banner = arg if arg
         @banner || description
       end
+
+      def block
+        @block || instance_method(:process) && instance_method(:process)
+      end
     end
 
     # Make those properties accessible to instances

test/test_command.rb

@@ -618,5 +618,9 @@ describe "Pry::Command" do
      it "should allow creating custom sub-classes of Pry::Command" do
        mock_pry("my---test").should =~ /my-testmy-test/
      end
+
+     it "should show the source of the process method" do
+       mock_pry("show-command my-test").should =~ /output.puts command_name/
+     end
    end
  end