Basic impl for string-format; fixed generator

package.json

@@ -19,6 +19,7 @@
   },
   "dependencies": {
     "deref": "^0.2.5",
-    "faker": "^2.0.1"
+    "faker": "^2.0.1",
+    "randexp": "^0.4.0"
   }
 }

spec/core/types/strings.json

@@ -18,6 +18,14 @@
           "maxLength": 30
         },
         "valid": true
+      },
+      {
+        "description": "should handle pattern",
+        "schema": {
+          "type": "string",
+          "pattern": "\\w+@\\w+\\.\\w+"
+        },
+        "valid": true
       }
     ]
   }