Add _.countBy and related unit tests, benchmarks, and documentation.

Former-commit-id: 915eaf414883a1dc344a558b08c7a1337ab5a225

build/pre-compile.js

@@ -46,7 +46,8 @@
     'target',
     'thisArg',
     'toString',
-    'value'
+    'value',
+    'valueProp'
   ];
 
   /** Used to minify `compileIterator` option properties */
@@ -93,6 +94,7 @@
     'compact',
     'compose',
     'contains',
+    'countBy',
     'criteria',
     'debounce',
     'defaults',