Responsive Layout

Potentially breaking changes

• Added functionality for automatic resizing/fluid layout. The plot will now resize to fit the containing element, and automatically resize itself as that element resizes.
• PlotlyPlot.prototype.purge now acts asynchronously and returns a Promise, like the other methods

Non-breaking changes

• Added a PlotlyPlot.prototype.resize method to explicitly call the resize code.
• Documented incompatibility between plotly.js and shadow DOM to README
• Improvements in CI: automatically release to NPM, automatically update documentation