Change tbls diff exit code

[k1LoW]

Exit status is 0 if inputs are the same, 1 if different, 2 if trouble.

http://man7.org/linux/man-pages/man1/diff.1.html

I made tbls diff exit code the same as diff exit code.

[codecov]

Codecov Report

Merging #62 into master will increase coverage by 0.03%.
The diff coverage is 33.33%.

@@            Coverage Diff             @@
##           master      #62      +/-   ##
==========================================
+ Coverage   75.03%   75.07%   +0.03%     
==========================================
  Files           7        7              
  Lines        1406     1408       +2     
==========================================
+ Hits         1055     1057       +2     
  Misses        253      253              
  Partials       98       98

Impacted Files	Coverage Δ
output/md/md.go	67.8% <33.33%> (+0.24%)	⬆️

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