README.md: drop IE

After re-testing IE, I find it best not to suggest that any version as
supported.
Remove classList dependency

Add the compatCLAdd and comapCLToggle functions to replace classList.add
and classList.toggle.

This should make fixing IE possible.
CSS: .clicked, .bomb, and .flagged work together

Instead of replacing classes, add them, and make sure the combined
classes render correctly.
PenF: size viewBox more appropriately

The viewBox was originally set up to be good enough for development
work; this commit should make it good enough to play arbitrary board
sizes.

Disclaimer: most of the scaling numbers used came from holding a ruler up to
the screen, then tweaking the results.