fread support na.strings="0" and na.strings="1"

[ben-schwen]

Closes #2927

[codecov]

Codecov Report

Merging #5196 (ab7e6f1) into master (8f0afcb) will increase coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head ab7e6f1 differs from pull request most recent head 2d0ba3e. Consider uploading reports for the commit 2d0ba3e to get more accurate results

@@           Coverage Diff           @@
##           master    #5196   +/-   ##
=======================================
  Coverage   99.38%   99.38%           
=======================================
  Files          77       77           
  Lines       14540    14541    +1     
=======================================
+ Hits        14451    14452    +1     
  Misses         89       89           

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

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