Added concurrency support to the pretty formatter

[lonnblad]

#254

[codecov]

Codecov Report

Merging #275 into master will increase coverage by 0.41%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #275      +/-   ##
==========================================
+ Coverage   84.14%   84.56%   +0.41%     
==========================================
  Files          16       16              
  Lines        2460     2494      +34     
==========================================
+ Hits         2070     2109      +39     
+ Misses        271      266       -5     
  Partials      119      119              

Impacted Files	Coverage Δ
fmt.go	94.55% <100.00%> (+0.15%)	⬆️
fmt_pretty.go	93.52% <100.00%> (+2.90%)	⬆️
run.go	77.63% <100.00%> (-1.71%)	⬇️
suite_context.go	70.57% <100.00%> (+0.23%)	⬆️
suite.go	88.18% <0.00%> (+0.52%)	⬆️

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