This imply scanning and transforming the heading text.
As the heading text is transformed, we need to stor it somewhere.
Instead of doing almost one malloc for each heading, allocate a large
buffer and store all indentifier inside, and using index.

We also remember the heading in the header Block to later retrieve it.

Do not hold pointer to reallocated memory. Realloc can copy to a
new location and our pointers are now invalid.
memorize the offset in the buffer instead, this remain valid even after
reallochas relocated the buffer

the block line(s) is always trimed when build

like `ΑΓΩ` is changed to `αγω`

It works but need more work because
- bad O(n²) algorithm to be replaced by a hash[Map|Set]
- use snprintf for simple int_to_str implementation

…fier

 ISUNICODEPUNCT_() use a codepoint not an offset. use ISPUNCT_()

some indentation cleaning

Use a pointer and a size instead of begining and ending index.
We need a pointer because we want to store the heading
identifier as destination, but the identifier is not part of the
initial ctx->txt buffer. This is done like the `title`
We also cascade change the `MD_LINK_ATTR` struct and supporting functions

so we can reuse it for heading

+typo

- depth for toc output
- Increase the abi_version to 1
- add --table-of-content option to md2html
- add --toc-depth=x to limit TOC levels

--table-of -content option has a parameter to set the mark
--toc is a shorthand for the --table-of-content option

wrong TOC depth is now an error