Add beforeDelete transform

The id list is deliberately not replaced with the value returned from
the transform as the user should not be able to silently remove ids from
the id list. Therefore, this 'transform' should be used purely as a way
to throw errors when the requested deletion is not allowed.

Closes #66

src/ResourceTypeRegistry.js

@@ -5,7 +5,7 @@
  * whose property is being retrieved/set, and the value to set it to, if any.
  */
 const autoGetterSetterProps = ["dbAdapter", "beforeSave", "beforeRender",
-  "labelMappers", "defaultIncludes", "info", "parentType"];
+  "beforeDelete", "labelMappers", "defaultIncludes", "info", "parentType"];
 
 /**
  * To fulfill a JSON API request, you often need to know about all the resources

src/controllers/API.js

@@ -2,6 +2,7 @@ import co from "co";
 
 import Response from "../types/HTTP/Response";
 import Document from "../types/Document";
+import Resource from "../types/Resource";
 import Collection from "../types/Collection";
 import APIError from "../types/APIError";
 
@@ -114,6 +115,28 @@ class APIController {
           }
         }
 
+        if (request.method === "delete") {
+          let toTransform;
+
+          if (Array.isArray(request.idOrIds)) {
+            toTransform = new Collection(
+              request.idOrIds.map((id) => {
+                if (typeof id === "string") {
+                  return new Resource(request.type, id);
+                }
+              })
+            );
+          }
+
+          else if (typeof request.idOrIds === "string") {
+            toTransform = new Resource(request.type, request.idOrIds);
+          }
+
+          yield applyTransform(
+            toTransform, "beforeDelete", registry, frameworkReq, frameworkRes
+          );
+        }
+
         // Actually fulfill the request!
         // If we've already populated the primary resources, which is possible
         // because the label may have mapped to no id(s), we don't need to query.