Added the ENTITIES member to SAXParser.

The README is clear that the ENTITIES member is accessible to client
code and can be modified by client code as needed. See:

https://github.com/isaacs/sax-js/blob/v1.2.1/README.md#regarding-doctypes-and-entitys

sax/index.d.ts

@@ -1,4 +1,4 @@
-// Type definitions for sax js
+// Type definitions for sax-js 1.x
 // Project: https://github.com/isaacs/sax-js
 // Definitions by: Asana <https://asana.com>
 // Definitions: https://github.com/DefinitelyTyped/DefinitelyTyped
@@ -63,6 +63,7 @@ export declare class SAXParser {
     strict: boolean;
     opt: SAXOptions;
     tag: string;
+    ENTITIES: {[key: string]: string};
 
     // Events
     onerror(e: Error): void;

sax/sax-tests.ts

@@ -13,6 +13,8 @@ import fs = require("fs");
 
     let parser = sax.parser(/*strict=*/true, opts);
 
+    parser.ENTITIES["foo"] = "bar";
+
     parser.onerror = function(e: Error) {
     };