config: implement ncu-config

[joyeecheung]

Part of #106, but I think we could get this in first since it's a separate command.

[codecov]

Codecov Report

Merging #139 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #139   +/-   ##
=======================================
  Coverage   98.05%   98.05%           
=======================================
  Files          15       15           
  Lines         567      567           
=======================================
  Hits          556      556           
  Misses         11       11

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