Refactor pkg/name to use options

[jonjohnsonjr]

This changes the Strictness parameter to be a more general Option so
that we can also pass in name.Insecure for insecure registries.

Note that by changing the name.StrictValidation and name.WeakValidation
enums to be Options, this change avoids breaking callers or introducing
new methods.

Fixes #416

[codecov-io]

Codecov Report

Merging #419 into master will increase coverage by 0.12%.
The diff coverage is 96.87%.

@@            Coverage Diff             @@
##           master     #419      +/-   ##
==========================================
+ Coverage   54.17%   54.29%   +0.12%     
==========================================
  Files          87       88       +1     
  Lines        4118     4129      +11     
==========================================
+ Hits         2231     2242      +11     
  Misses       1545     1545              
  Partials      342      342

Impacted Files	Coverage Δ
pkg/name/check.go	100% <ø> (ø)	⬆️
pkg/name/registry.go	96% <100%> (-0.16%)	⬇️
pkg/name/tag.go	80.64% <100%> (+0.64%)	⬆️
pkg/name/options.go	100% <100%> (ø)
pkg/name/digest.go	77.77% <100%> (ø)	⬆️
pkg/name/ref.go	100% <100%> (ø)	⬆️
pkg/name/repository.go	80% <75%> (+0.58%)	⬆️

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