Add support for multiple media types inside content component

[melvinitcr]

Added support for diff changes when there are multiple
media types inside a content component.

The diff changes include: added, deleted and modified
media types.

All modified media types are now indexed by the media type
name. This adds one extra element to the component hierarchy.

Updated README.md file and unit test accordingly.

[codecov-commenter]

Codecov Report

Merging #47 (819a5f3) into main (fde406a) will decrease coverage by 0.58%.
The diff coverage is 84.00%.

@@            Coverage Diff             @@
##             main      #47      +/-   ##
==========================================
- Coverage   92.68%   92.09%   -0.59%     
==========================================
  Files          63       64       +1     
  Lines        2842     2860      +18     
==========================================
  Hits         2634     2634              
- Misses        143      155      +12     
- Partials       65       71       +6     

Flag	Coverage Δ
unittests	92.09% <84.00%> (-0.59%)	⬇️

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

Impacted Files	Coverage Δ
report/report.go	79.86% <57.14%> (-1.17%)	⬇️
diff/content_diff.go	93.02% <89.28%> (+1.35%)	⬆️
diff/media_type_diff.go	90.90% <90.90%> (ø)
diff/parameter_diff.go	75.00% <0.00%> (-12.50%)	⬇️
diff/method_diff.go	90.00% <0.00%> (-5.00%)	⬇️
diff/parameters_diff.go	87.90% <0.00%> (-2.42%)	⬇️

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[reuvenharrison]

Great stuff - thanks for your contribution!