👌 IMPROVE: Make link functions overridable

[chrisjsewell]

validateLink, normalizeLink and normalizeLinkText are now methods of MarkdownIt,
meaning that they can be overriden by subclassing MarkdownIt.

[codecov]

Codecov Report

Merging #135 (45bf6e0) into master (a40f6a8) will decrease coverage by 0.02%.
The diff coverage is 96.29%.

@@            Coverage Diff             @@
##           master     #135      +/-   ##
==========================================
- Coverage   96.19%   96.16%   -0.03%     
==========================================
  Files          72       72              
  Lines        3104     3107       +3     
==========================================
+ Hits         2986     2988       +2     
- Misses        118      119       +1     

Flag	Coverage Δ
pytests	96.16% <96.29%> (-0.03%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
markdown_it/rules_block/state_block.py	97.84% <66.66%> (-0.70%)	⬇️
markdown_it/common/normalize_url.py	95.23% <100.00%> (ø)
markdown_it/main.py	91.47% <100.00%> (+0.41%)	⬆️
markdown_it/rules_block/reference.py	97.65% <100.00%> (-0.02%)	⬇️
markdown_it/rules_core/linkify.py	97.70% <100.00%> (-0.03%)	⬇️
markdown_it/rules_inline/autolink.py	95.83% <100.00%> (-0.09%)	⬇️
markdown_it/rules_inline/image.py	98.86% <100.00%> (-0.02%)	⬇️
markdown_it/rules_inline/link.py	98.83% <100.00%> (-0.02%)	⬇️

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