highlight: add HighlightVisitor::line_id_prefix to control ids of line spans

Signed-off-by: Jean Privat <jean@pryen.org>
nitodoc: stop generating id for lines in examples

Signed-off-by: Jean Privat <jean@pryen.org>
nitlight: add option --no-line-id

Signed-off-by: Jean Privat <jean@pryen.org>
man: add --no-line-id to nitlight.md

Signed-off-by: Jean Privat <jean@pryen.org>
lib/opts: do not crash for empty arguments

Signed-off-by: Jean Privat <jean@pryen.org>