added `before_eval` hook & tests

This came about because KINGSABRI wanted his pry console to only execute commands, not ruby code (lol).
I conconcated the following piece of code that achieves this, but it required a before_eval hook:

```
_pry_.hooks.add_hook(:before_eval, :kill_ruby) do |code, pry|
  if !code.empty?
    pry.suppress_output = true
    pry.output.puts "Command #{code.strip} not found, type 'help'"
    code.replace("")
  end
end
```

lib/pry/pry_instance.rb

@@ -49,6 +49,8 @@ class Pry
 
   attr_accessor :extra_sticky_locals
 
+  attr_accessor :suppress_output
+
   # This is exposed via Pry::Command#state.
   attr_reader :command_state
 
@@ -268,6 +270,8 @@ class Pry
 
     code = r(target)
 
+    exec_hook :before_eval, code, self
+
     result = target.eval(code, Pry.eval_path, Pry.current_line)
     set_last_result(result, target, code)
 

test/test_hooks.rb

@@ -394,6 +394,37 @@ describe Pry::Hooks do
         Pry.config.exception_whitelist = old_ew
       end
 
+      describe "before_eval hook" do
+        describe "modifying input code" do
+          it 'should replace input code with code determined by hook' do
+            hooks = Pry::Hooks.new.add_hook(:before_eval, :quirk) { |code, pry| code.replace(":little_duck") }
+            redirect_pry_io(InputTester.new(":jemima", "exit-all"), out = StringIO.new) do
+              Pry.start(self, :hooks => hooks)
+            end
+            out.string.should =~ /little_duck/
+            out.string.should.not =~ /jemima/
+          end
+
+          it 'should not interfere with command processing when replacing input code' do
+            commands = Pry::CommandSet.new do
+              import_from Pry::Commands, "exit-all"
+
+              command "how-do-you-like-your-blue-eyed-boy-now-mister-death" do
+                output.puts "in hours of bitterness i imagine balls of sapphire, of metal"
+              end
+            end
+
+            hooks = Pry::Hooks.new.add_hook(:before_eval, :quirk) { |code, pry| code.replace(":little_duck") }
+            redirect_pry_io(InputTester.new("how-do-you-like-your-blue-eyed-boy-now-mister-death", "exit-all"), out = StringIO.new) do
+              Pry.start(self, :hooks => hooks, :commands => commands)
+            end
+            out.string.should =~ /in hours of bitterness i imagine balls of sapphire, of metal/
+            out.string.should.not =~ /little_duck/
+          end
+        end
+
+      end
+
       describe "exceptions" do
         before do
           Pry.config.hooks.add_hook(:after_eval, :baddums){ raise "Baddums" }