Add conditions to transactions

[garious]

Some of TODOs and missing tests here, but ready for a concept review.

[codecov-io]

Codecov Report

Merging #56 into master will decrease coverage by 2.17%.
The diff coverage is 85.03%.

@@            Coverage Diff             @@
##           master      #56      +/-   ##
==========================================
- Coverage    94.7%   92.52%   -2.18%     
==========================================
  Files          12       12              
  Lines         548      696     +148     
==========================================
+ Hits          519      644     +125     
- Misses         29       52      +23

Impacted Files	Coverage Δ
src/lib.rs	100% <ø> (ø)	⬆️
src/event.rs	40% <0%> (-60%)	⬇️
src/accountant.rs	95.54% <96.8%> (+1.04%)	⬆️
src/transaction.rs	96.29% <97.14%> (-0.97%)	⬇️
src/logger.rs	96.87% <0%> (-3.13%)	⬇️
src/accountant_stub.rs	90.58% <0%> (+1.17%)	⬆️

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