measure coverage of doctests too

[codecov[bot]]

Codecov Report

Merging #109 into master will increase coverage by 6.15%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #109      +/-   ##
==========================================
+ Coverage   79.28%   85.43%   +6.15%     
==========================================
  Files          24       25       +1     
  Lines        1752     1867     +115     
==========================================
+ Hits         1389     1595     +206     
+ Misses        363      272      -91     

Impacted Files	Coverage Δ
src/extension/itunes/itunes_owner.rs	100.00% <0.00%> (ø)
src/extension/itunes/itunes_category.rs	100.00% <0.00%> (ø)
src/lib.rs	100.00% <0.00%> (ø)
src/guid.rs	100.00% <0.00%> (+7.69%)	:arrow_up:
src/source.rs	100.00% <0.00%> (+8.33%)	:arrow_up:
src/category.rs	100.00% <0.00%> (+8.33%)	:arrow_up:
src/enclosure.rs	97.36% <0.00%> (+9.48%)	:arrow_up:
src/channel.rs	88.92% <0.00%> (+10.02%)	:arrow_up:
src/cloud.rs	98.07% <0.00%> (+11.41%)	:arrow_up:
src/textinput.rs	93.18% <0.00%> (+11.60%)	:arrow_up:
... and 4 more

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