Added support for custom object formatters

Custom object formatters allow users to customize how an object is
stringified in matcher failure messages. This can already be done by
adding a `jasmineToString` method to the objects in question. But
it's not always desirable or possible to do that, particularly when
objects of a given "type" do not inherit from a specific prototype.
For instance, suppose a web service returns a list of foos that are
deserialized from JSON, e.g.:

   { fooId: 42, /* more properties */ }

The only way to define `jasmineToString` on those is by writing code to
add it to each instance at runtime. But a custom object formatter can
recognize that the object it's looking at is a foo and format it
accordingly:

   jasmine.addCustomObjectFormatter(function(obj) {
      if (typeof obj.fooId !== 'number') {
            return undefined;
        }

        return '[Foo with ID ' + obj.fooId + ']';
    });

Unlike `jasmineToString`, custom object formatters are scoped to a
particular spec or suite and don't require any changes to the code
under test.

src/core/PrettyPrinter.js

@@ -1,9 +1,11 @@
 getJasmineRequireObj().makePrettyPrinter = function(j$) {
-  function SinglePrettyPrintRun() {
+  function SinglePrettyPrintRun(customObjectFormatters, pp) {
+    this.customObjectFormatters_ = customObjectFormatters;
     this.ppNestLevel_ = 0;
     this.seen = [];
     this.length = 0;
     this.stringParts = [];
+    this.pp_ = pp;
   }
 
   function hasCustomToString(value) {
@@ -24,7 +26,11 @@ getJasmineRequireObj().makePrettyPrinter = function(j$) {
   SinglePrettyPrintRun.prototype.format = function(value) {
     this.ppNestLevel_++;
     try {
-      if (j$.util.isUndefined(value)) {
+      var customFormatResult = this.applyCustomFormatters_(value);
+
+      if (customFormatResult) {
+        this.emitScalar(customFormatResult);
+      } else if (j$.util.isUndefined(value)) {
         this.emitScalar('undefined');
       } else if (value === null) {
         this.emitScalar('null');
@@ -33,7 +39,7 @@ getJasmineRequireObj().makePrettyPrinter = function(j$) {
       } else if (value === j$.getGlobal()) {
         this.emitScalar('<global>');
       } else if (value.jasmineToString) {
-        this.emitScalar(value.jasmineToString());
+        this.emitScalar(value.jasmineToString(this.pp_));
       } else if (typeof value === 'string') {
         this.emitString(value);
       } else if (j$.isSpy(value)) {
@@ -95,6 +101,18 @@ getJasmineRequireObj().makePrettyPrinter = function(j$) {
     }
   };
 
+  SinglePrettyPrintRun.prototype.applyCustomFormatters_ = function(value) {
+    var i, result;
+
+    for (i = 0; i < this.customObjectFormatters_.length; i++) {
+      result = this.customObjectFormatters_[i](value);
+
+      if (result !== undefined) {
+        return result;
+      }
+    }
+  };
+
   SinglePrettyPrintRun.prototype.iterateObject = function(obj, fn) {
     var objKeys = keys(obj, j$.isArray_(obj));
     var isGetter = function isGetter(prop) {};
@@ -365,11 +383,16 @@ getJasmineRequireObj().makePrettyPrinter = function(j$) {
     return extraKeys;
   }
 
-  return function() {
-    return function(value) {
-      var prettyPrinter = new SinglePrettyPrintRun();
+  return function(customObjectFormatters) {
+    var pp = function(value) {
+      var prettyPrinter = new SinglePrettyPrintRun(
+        customObjectFormatters || [],
+        pp
+      );
       prettyPrinter.format(value);
       return prettyPrinter.stringParts.join('');
     };
+
+    return pp;
   };
 };