Add plugins

[gonzofish]

Addresses #36

What was changed

• Added a files utility to load a module (loadModule)
• Added a config option for plugins, which is an Array<string> where each item is a Node package name
• Change the unknown command fallback to check plugins for a matching command first

[codecov-commenter]

Codecov Report

Base: 100.00% // Head: 100.00% // No change to project coverage 👍

Coverage data is based on head (bbc728a) compared to base (5d5eb84).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff            @@
##            master       #37   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           16        16           
  Lines          555       579   +24     
=========================================
+ Hits           555       579   +24     

Impacted Files	Coverage Δ
lib/utils/files.util.js	100.00% <ø> (ø)
lib/index.js	100.00% <100.00%> (ø)
lib/utils/config.util.js	100.00% <100.00%> (ø)

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