Script update for references and checks

[jmb]

Re-run of PR #770 with hopefully more success! 😄

This PR updates the update_readme.py to keep a REFERENCES.md and a CHECK.html file updated.
A working example of the check html file is here: https://jmb.github.io/SuperTinyIcons/CHECK.html - it shows the SVG icon with border=50% so that any that don't fit the circle are easily recognised.

It currently looks for reference images with the same base name as the svg, then checks the images/reference/index.md for the source location and any other reference images that might have a different name. Eventually this check should probably be moved to look at the REFERENCES.md file and the index.md removed.