introduced BlockNode#node_id and methods that assign values of the id
abandoned the change introduced by commit 9d2b6ce. It was a wrong approach that alters visited trees
introduced BlockLeaf#node_id
fixed a bug that wrongly assigns NestedBlockLeaf#node_id
introduced HtmlFormat::HeadingNodeFormatter
reimplemented the additional feature for HtmlFormat that allow to assign ids to headings
Merge branch 'develop' into 2html

fixed a bug that alters visited trees with '[leaf_id]' at the head of HeadingLeaf or ListTypeLeaf