Added Kaleido for exporting static images from Plotly JSON.
Everything is taken care of, but this still needs to be tested on Mac OS and Linux, and perhaps also on an older MATLAB version.

Simple usage:

write_image(<plotly figure object>);

plotly figure object is returned by the fig2plotly() function.

Complete usage:

write_image(<plotly figure object>, <image format>, <filename>, <image height>, <image width>, <image scaling factor>);

image format can be any of the following: "png", "jpg", "jpeg", "webp", "svg", "pdf", "eps", "json"

Example code:

[X,Y,Z] = peaks;
contour(X,Y,Z);
fig=fig2plotly(gcf);
write_image(fig);

You should have a new file: figure.png in your working directory.

Note reviewers: If you plan on providing more than 1 argument to this function, please provide them all. I will change this behavior tomorrow. Until then, please try it and let me know your comments/suggestions/improvement ideas, etc.