add selector to BlockIO classes

[shannonwells]

closes #169

[codecov-io]

Codecov Report

Merging #178 into master will decrease coverage by 0.09%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #178      +/-   ##
==========================================
- Coverage   68.55%   68.46%   -0.08%     
==========================================
  Files          36       37       +1     
  Lines        1987     1991       +4     
==========================================
+ Hits         1362     1363       +1     
- Misses        528      530       +2     
- Partials       97       98       +1     

Impacted Files	Coverage Δ
retrievalmarket/impl/blockio/reader.go	71.43% <100.00%> (ø)
retrievalmarket/impl/blockio/traverser.go	70.24% <100.00%> (-3.87%)	⬇️
retrievalmarket/impl/blockio/verify.go	76.48% <100.00%> (ø)
retrievalmarket/impl/client.go	67.70% <100.00%> (ø)
retrievalmarket/impl/common.go	100.00% <100.00%> (ø)
retrievalmarket/impl/provider.go	70.67% <100.00%> (ø)

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[hannahhoward]

LGTM