- writepage() improve error messages for filenames (error on line ...).
	show template name and filename.
- improve documentation

make template structure:

	templates/name/header.ext
	templates/name/item.ext
	templates/name/footer.ext

add able to check unused variables (-w option?).
	add used counter to variable struct.

must work on: Linux, OpenBSD, MingW (Windows).
	glibc
	musl
	OpenBSD libc

- show line number in error
	improve
- about 500 LOC
