Provide an option to return base64-encoded images in CI log

[astrofrog]

Some CI services don't have good support for artifacts. In those cases we could print out to the log the base64 expected and actual image, inside a script to generate these files. Then one can simply copy/paste the script, run it locally, and get the images. See https://github.com/WorldWideTelescope/pywwt/blob/master/pywwt/tests/test_qt_widget.py#L67 for an example of this.

This could be triggered e.g. by an environment variable or a command-line flag (--mpl-base64).

Opened this following mention of this problem in #81