feat: add autoUnref option

With autoUnref set to true (default: false), the Engine.IO client will
allow the program to exit if there is no other active timer/socket in
the event system.

Note: the 'xmlhttprequest-ssl' package has been copied in the contrib/
directory, until the change is merged upstream

Related: #653

README.md

@@ -262,6 +262,7 @@ Exposed as `eio` in the browser standalone build.
       - `onlyBinaryUpgrades` (`Boolean`): whether transport upgrades should be restricted to transports supporting binary data (`false`)
       - `forceNode` (`Boolean`): Uses NodeJS implementation for websockets - even if there is a native Browser-Websocket available, which is preferred by default over the NodeJS implementation. (This is useful when using hybrid platforms like nw.js or electron) (`false`, NodeJS only)
       - `localAddress` (`String`): the local IP address to connect to
+      - `autoUnref` (`Boolean`): whether the transport should be `unref`'d upon creation. This calls `unref` on the underlying timers and sockets so that the program is allowed to exit if they are the only timers/sockets in the event system (Node.js only)
     - **Polling-only options**
       - `requestTimeout` (`Number`): Timeout for xhr-polling requests in milliseconds (`0`)
     - **Websocket-only options**