Implement SimpleBlockReader

[at-wat]

Taking over #129

[codecov]

Codecov Report

Merging #150 (9f4c708) into master (d967435) will increase coverage by 0.06%.
The diff coverage is 96.58%.

@@            Coverage Diff             @@
##           master     #150      +/-   ##
==========================================
+ Coverage   99.15%   99.21%   +0.06%     
==========================================
  Files          23       24       +1     
  Lines        1308     1409     +101     
==========================================
+ Hits         1297     1398     +101     
  Misses          6        6              
  Partials        5        5              

Impacted Files	Coverage Δ
mkvcore/blockwriter.go	98.37% <50.00%> (-0.02%)	⬇️
mkvcore/blockreader.go	97.61% <97.61%> (ø)
mkvcore/option.go	100.00% <100.00%> (ø)
unmarshal.go	100.00% <100.00%> (+1.15%)	⬆️

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