add default_auth

[daddygongon]

Thanks for the useful gem. I really use it pretty often. Now I will nbconvert the notebook to operate batch jobs, the Plotly.show should be necessary to change the download_image. Although the README recommends to use Plotly.auth each time, I think it is more convenient to use the default credential file for Plotly. Thus I added,

• default_auth
• set_credentials_file in client.rb.

Hope to accept the pull request.

Usage

• Plotly.set_credentials_file(user_name, user_token)
• Plotly.default_auth