Added filesListFolderContinue example

[allenl7]

@greg-db https://github.com/dropbox/dropbox-sdk-js/issues/14#issuecomment-239939720

Added filesListFolderContinue example as suggested by Greg.

[greg-db]

Thanks!

[rogebrd]

Hello, thank you for your PR! We would love to include this example. However, we just released a major version that updated a lot of the examples. If you wouldn't mind rebasing and updating, I would love to merge this in.

[CLAassistant]

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