Adding newly added PoolOptions (#16)

The following [PR](sidorares/node-mysql2#1081) added two new options to mysql's Pool creation.

I am simply adding them to the library in this PR.

lib/Pool.d.ts

@@ -31,6 +31,17 @@ declare namespace Pool {
          * is no limit to the number of queued connection requests. (Default: 0)
          */
         queueLimit?: number;
+
+        /**
+         * Enable keep-alive on the socket.  It's disabled by default, but the
+         * user can enable it and supply an initial delay.
+         */
+        enableKeepAlive?: true;
+
+        /**
+         * If keep-alive is enabled users can supply an initial delay.
+         */
+        keepAliveInitialDelay?: number;
     }
 }