Set app.jinja_env.undefined to something better when debug = True

Currently missing attributes of template variables are swallowed by Flask, even
in debug mode:

```
@app.route('/')
def index():
    # template contains:
    # {{ lol.asdf }}
    return flask.render_template('lol.htm', lol=3)
```

I propose we set `app.jinja_env.undefined` to some type that logs warnings to
console when Flask's debug mode is enabled.
