Add new attributes for subscriptions

[cre8ivejp]

fix #6

[codecov-io]

Codecov Report

Merging #7 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##             master     #7   +/-   ##
=======================================
  Coverage       100%   100%           
- Complexity      264    270    +6     
=======================================
  Files            22     22           
  Lines           808    838   +30     
  Branches         49     49           
=======================================
+ Hits            808    838   +30

Impacted Files	Coverage Δ	Complexity Δ
.../src/main/java/jp/alessandro/android/iab/Item.java	100% <100%> (ø)	20 <7> (+6)	⬆️

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