Allow user to customize generated HTML

[strager]

md_html allows very little customization. If users want to use <tt>
instead of <code>, for example, they need to reimplement md_html
entirely or post-process md_html's output.

Expose some of md_html's internals to make it easier for users to
customize md_html's output.

[codecov]

Codecov Report

Merging #158 (5d4c578) into master (da77b92) will decrease coverage by 1.47%.
The diff coverage is 30.55%.

@@            Coverage Diff             @@
##           master     #158      +/-   ##
==========================================
- Coverage   94.46%   92.98%   -1.48%     
==========================================
  Files           3        4       +1     
  Lines        3088     3151      +63     
==========================================
+ Hits         2917     2930      +13     
- Misses        171      221      +50     

Impacted Files	Coverage Δ
src/md4c-html-example.c	0.00% <0.00%> (ø)
src/md4c-html.c	94.36% <88.00%> (-0.84%)	⬇️

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[mity]

Hello, 1st of all sorry for the very late response to this.

I find this as an interesting idea generally, but it needs some more work before I'm ready to merge this.

1. Most importantly, the new interface in md4c-html.h would have to be re-designed in some bigger way to allow accommodating future expansion of md4c.h interface.

Imagine future version of the structure MD_PARSER gets a new (optional) member which allows using a new optional feature. The parser (md4c.c) would only use the new member and enable the feature only when MD_PARSER::abi_version >= 1 (for compatibility with current applications out there.) Completely legitimate thing to do.

But such future feature then could never be used together with md_html_create() because we could not change interface of that function so it could not be able to initialize the new struct member(s).

2. Please avoid using C-99 and C-11 struct initializers in the code. In some cases MD4C is translated with very antique C compilers (for example Qt uses MD4C and they need to compile it for some embedded systems with obscure old C compiler).

3. I would also prefer to move md4c-html-example.c to a new dir (e.g. md4c/examples) to not clutter the main stuff. Example serves both partly a feature of source code and partly also that of a documentation so it would probably be better from that perspective too.

[strager]

I don't plan to work on this patch. Feel free to take it over. 👍