BREAKING: Add collaborative render postprocessors for plugins. Input new RenderContext to both renderers and postprocessors

[hukkin]

One possible solution to #174

[codecov]

Codecov Report

Merging #175 (fd95a9d) into master (8066530) will increase coverage by 1.54%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #175      +/-   ##
==========================================
+ Coverage   95.05%   96.60%   +1.54%     
==========================================
  Files          14       14              
  Lines         850      854       +4     
==========================================
+ Hits          808      825      +17     
+ Misses         42       29      -13     

Impacted Files	Coverage Δ
mdformat/codepoints/_unicode_punctuation.py	100.00% <ø> (+75.00%)	⬆️
mdformat/codepoints/_unicode_whitespace.py	100.00% <ø> (+75.00%)	⬆️
mdformat/plugins.py	100.00% <100.00%> (ø)
mdformat/renderer/__init__.py	98.07% <100.00%> (+0.11%)	⬆️
mdformat/renderer/_default_renderers.py	98.57% <100.00%> (+0.01%)	⬆️
mdformat/renderer/_tree.py	95.59% <100.00%> (+1.00%)	⬆️
mdformat/renderer/typing.py	80.00% <100.00%> (+5.00%)	⬆️

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