Config argument for baseUrl

[chriddyp]

Right now, the base url for the /external endpoint is taken from the window. Could we add an option to the config as well? That'd make it easier for Dash users to point to their On-Premise instances:

plotly.js/src/plots/plots.js

Line 217 in e7896e9

var baseUrl = (window.PLOTLYENV && window.PLOTLYENV.BASE_URL) || 'https://plot.ly';

something like

 var baseUrl = (
    (window.PLOTLYENV && window.PLOTLYENV.BASE_URL) || 
    config.plotly_server_url ||
    'https://plot.ly';

cc @cldougl

[etpinard]

Sure. 🐫 that config option to plotlyServerUrl and PR away 🚀