Fread sep block dec

[MichaelChirico]

Closes #4483

[codecov]

Codecov Report

Merging #4495 (add2f9d) into master (01da119) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #4495   +/-   ##
=======================================
  Coverage   99.54%   99.54%           
=======================================
  Files          76       76           
  Lines       14611    14612    +1     
=======================================
+ Hits        14544    14545    +1     
  Misses         67       67           

Impacted Files	Coverage Δ
src/fread.c	99.40% <100.00%> (+<0.01%)	⬆️

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