Enable all V8-implemented Harmony features in Node.js

E.g., to start the Node.js REPL with all harmony features enabled, run the following:

$ node `node --v8-options | grep harmony | cut -d ' ' -f | xargs`

You can now play around with some of the harmony features already implemented in V8. E.g., try the following:

> let wm = new WeakMap();
> let key = {};
> wm.set(key, 'foo');
> wm.get(key);
'foo'