Allow node client to read ~/.plotly/.credentials

[ryan-williams]

Re-filing plotly/plotly.js#1386 here.

It seems that the only way to auth with the node client is to put your username and api-key (effectively password) into your source (docs).

It would be better if there were an option to have it read credentials from ~/.plotly/.credentials, like the python client does.

I'm trying to ship a script that users can run to generate a plot with data of their choosing; putting my own credentials into that script / my SCM is not desirable or sensible.

Perhaps I've missed a way to do this currently? As it is, I'm going to parse ~/.plotly/.credentials myself in JS, and pass the u/p to require('plotly').