Allow tag and digest

[viceice]

fixes #351

[codecov-io]

Codecov Report

Merging #391 into master will increase coverage by 0.03%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #391      +/-   ##
==========================================
+ Coverage   51.63%   51.67%   +0.03%     
==========================================
  Files          77       77              
  Lines        3693     3696       +3     
==========================================
+ Hits         1907     1910       +3     
  Misses       1497     1497              
  Partials      289      289

Impacted Files	Coverage Δ
pkg/name/digest.go	78.57% <100%> (+2.57%)	⬆️

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[jonjohnsonjr]

We might want to extend this to store an optional tag in the digest strcut so that NewDigest(str).String() == str but I think this is fine for now. Thanks!