Add an update check command to check for updates.

[marcomorain]

No description provided.

[codecov-io]

Codecov Report

Merging #26 into master will decrease coverage by 1.52%.
The diff coverage is 23.33%.

@@            Coverage Diff             @@
##           master      #26      +/-   ##
==========================================
- Coverage    41.6%   40.08%   -1.53%     
==========================================
  Files          11       12       +1     
  Lines         661      721      +60     
==========================================
+ Hits          275      289      +14     
- Misses        363      409      +46     
  Partials       23       23

Impacted Files	Coverage Δ
cmd/version.go	57.14% <ø> (ø)	⬆️
cmd/root.go	75.47% <100%> (+0.47%)	⬆️
cmd/update.go	22.03% <22.03%> (ø)

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[eric-hu]

Pulled and verified the circleci update check command displays the latest release on Github.

LGTM