Stop the watcher from reacting to directory events

[cjohansen]

On OSX changing files will often also emit events on the containing directory,
causing excessive test runs while in --watch mode. This manifests as duplicate
test runs, but is especially visible when ignoring certain file patterns.

This commit adds a check for directories and ignores those events.

[codecov]

Codecov Report

Merging #63 into master will increase coverage by 0.04%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #63      +/-   ##
==========================================
+ Coverage   77.47%   77.52%   +0.04%     
==========================================
  Files          40       40              
  Lines        1896     1900       +4     
  Branches      161      161              
==========================================
+ Hits         1469     1473       +4     
  Misses        337      337              
  Partials       90       90

Flag	Coverage Δ
#integration	57.63% <20%> (-0.07%)	⬇️
#unit	74.73% <100%> (+0.05%)	⬆️

Impacted Files	Coverage Δ
src/kaocha/core_ext.clj	78.78% <100%> (+1.36%)	⬆️
src/kaocha/watch.clj	78.43% <100%> (+0.28%)	⬆️

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[plexus]

Thank you!