Bascule Attributes should supported key path getters

[joe94]

This PR introduces breaking changes to the library to add support for getting values from a token attribute given a key path.

Todo:

• Unit tests

Note: the reason why we need to do an additional casting error check is that viper does not provide it 😢 https://github.com/spf13/cast#usage

[codecov-io]

Codecov Report

Merging #52 into master will decrease coverage by 0.26%.
The diff coverage is 74.73%.

@@            Coverage Diff             @@
##           master      #52      +/-   ##
==========================================
- Coverage   76.33%   76.07%   -0.27%     
==========================================
  Files          25       25              
  Lines         731      815      +84     
==========================================
+ Hits          558      620      +62     
- Misses        155      167      +12     
- Partials       18       28      +10

Impacted Files	Coverage Δ
checks.go	95.65% <100%> (-4.35%)	⬇️
basculehttp/tokenFactory.go	30.43% <33.33%> (ø)	⬆️
token.go	79.38% <77.27%> (-20.62%)	⬇️

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[kristinapathak]

Thanks for adding this - I can't wait to use it! 😄

[johnabass]

Looks good to me!

[joe94]

fixes #51