This functionality is being added in #1234 using a slightly different approach. With this PR, the include_plotlyjs argument to plotly.offline.plot accepts a few options in addition to True and False. In particular, setting include_plotlyjs='directory' results in the behavior of this PR (a copy of plotly.min.js is copied to the output directory if it does not already exist, and the script tag references this local copy).

I'd like to avoid having this be a one-off global flag for the time being. Eventually I hope we'll be able to revamp our settings system a bit to make it possible to register default values for options like include_plotlyjs.

Thanks for your work on this @fu, the include_plotlyjs='directory' option wouldn't have been included in #1234 without it!