[WIP, Experimental] Runtime customization by application side.

[mity]

This PR is an attempt to implement the proposal discussed in #245.

Rough to-do list:

• [x] Change API in md4c.h so that the callbacks can get custom block/span/text type IDs.
• [x] Change API in md4c-html.h so that application can override the output of the HTML renderer for some block/span/text types.
• [ ] Expand MD_PARSER so that it's possible to add support for new custom span ranges.
• [ ] Implement such custom expansion in md2html (e.g. subscript, superscript)
• [ ] Expand MD_PARSER so that it's possible to add support for new custom auto-ending spans (name-like).
• [ ] Implement such custom expansion in md2html (e.g. #183 and/or user mentions).

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 83.00000% with 17 lines in your changes are missing coverage. Please review.

Project coverage is 91.44%. Comparing base (3848bfb) to head (3e93dd5).

Files	Patch %	Lines
src/md4c-html.c	87.95%	10 Missing :warning:
src/md4c.c	58.82%	7 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #249 +/- ## ========================================== - Coverage 91.74% 91.44% -0.30% ========================================== Files 5 5 Lines 3366 3389 +23 Branches 764 765 +1 ========================================== + Hits 3088 3099 +11 - Misses 278 290 +12 ```

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