So, here's a patch I would like a couple of people to try before i
commit it. It provides customization of checkbox formatting for html
output (unicode, ascii, checkbox input fields or custom). I couldn't
figure out how to add an association list customization with preset
options to a customization choice, so the custom option is a
partilally pre-filled sexp.

The default is still ascii, partially so that the checkbox tests don't
fail w/o being changed.

Let me know if this works for every of if it is too complex a solution
befor I apply it to master.

tia
rick