add config.getDir, loads all files in a directory

[msimerson]

this is in preparation for haraka/Haraka#1828

• api for getDir is a node callback
• unlike get, getDir is async and returns nothing. The results are in the callback.
• internally, getDir is implemented entirely with promises
• dir watching

[codecov-io]

Codecov Report

Merging #18 into master will increase coverage by 2.27%.
The diff coverage is 97.77%.

@@            Coverage Diff             @@
##           master      #18      +/-   ##
==========================================
+ Coverage   77.38%   79.65%   +2.27%     
==========================================
  Files           7        7              
  Lines         367      408      +41     
  Branches      106      114       +8     
==========================================
+ Hits          284      325      +41     
  Misses         83       83

Impacted Files	Coverage Δ
configfile.js	67.48% <100%> (+6.54%)	✅
readers/binary.js	100% <100%> (ø)	✅
config.js	91.54% <80%> (+0.24%)	✅

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