Allow up to 6 levels of headers in the table of content and optional ignoring of h1 headers

[oliora]

Allow up to 6 levels of headers in the table of content and allow to ignore h1 header

Any number of levels is supported but since the official HTML supports only 6 levels (h1 ... h6) an artifical limitation was added.

This change affects two options: toc_level and ordered_chapter_level. Both can be set to a value between 0 and 6 (inclusive) where 0 means the feature is disabled.

If either of the options is set to a value greater than 6 then 6 levels of headers are processed and a warning is emitted.

This change also adds option ignore_top_header to exclude h1 headers from the table of content and numbering.