Added an apply method to Prettifier that takes two Any arguments and returns

a PrettyPair, which I also just added. PrettyPair has two prettifier objects, which
were found to be unequal, so the idea is that this new apply method on Prettifier
will use the differences to help decide how to prettify them. For example in strings
we could put the []'s in the right spots. Or for long strings we can shorten them
and highlight just the part (or at least the first part) that is different. And
there's also a hint string in PrettyPair, that can be used to give info about
the difference.

Author: bvenners

scalactic-test/src/test/scala/org/scalactic/PrettifierSpec.scala

@@ -54,6 +54,19 @@ class PrettifierSpec extends FunSpec with Matchers {
       myLittlePretty(()) should be ("<(), the Unit value>")
       myLittlePretty(List("1", "2", "3")) should be ("List(\"1\", \"2\", \"3\")")
     }
+    it("should by default offer an apply method that takes two args that returns a PrettyPair whose left and right are prettified by the one-arg apply and hint is None.") {
+    
+      case class Yell(secret: String)
+      val myLittlePretty =
+        new Prettifier {
+          def apply(o: Any) =
+            o match {
+              case Yell(secret) => secret.toUpperCase + "!!!"
+              case _ => Prettifier.default(o)
+            }
+        }
+      myLittlePretty(Yell("I like fruit loops"), Yell("I like raisin bran")) shouldBe PrettyPair("I LIKE FRUIT LOOPS!!!", "I LIKE RAISIN BRAN!!!", None)
+    }
   }
 
   describe("the basic Prettifier") {

scalactic/src/main/scala/org/scalactic/Prettifier.scala

@@ -126,6 +126,13 @@ trait Prettifier extends Serializable { // I removed the extends (Any => String)
    * Prettifies the passed object.
    */
   def apply(o: Any): String
+
+  def apply(left: Any, right: Any): PrettyPair =
+    PrettyPair(
+      apply(left),
+      apply(right),
+      None
+    )
 }
 
 /**

scalactic/src/main/scala/org/scalactic/PrettyPair.scala

@@ -0,0 +1,25 @@
+/*
+ * Copyright 2001-2016 Artima, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.scalactic
+
+case class PrettyPair(
+  left: String,
+  right: String,
+  hint: Option[String]
+)
+
+
+