Use -o to overwrite images.

This means I no longer need to delete the entire output directory before regenerating the HTML format output.
Use the new stylesheet.
Better naming for the main loop.
lock_picked would imply the lock has been broken, but this routine is called while it's being picked so rename it to picking_a_lock.
Tidy-up some of the user input documentation.
Remove clumsy overlap.$xxxx syntax for overlapping items. Just use $xxxx.