Add CloudFront cache invalidation

[bep]

Fixes #31

[codecov-io]

Codecov Report

Merging #32 into master will decrease coverage by 2.36%.
The diff coverage is 54.23%.

@@            Coverage Diff             @@
##           master      #32      +/-   ##
==========================================
- Coverage   69.95%   67.59%   -2.37%     
==========================================
  Files           6        8       +2     
  Lines         436      577     +141     
==========================================
+ Hits          305      390      +85     
- Misses        103      157      +54     
- Partials       28       30       +2

Impacted Files	Coverage Δ
lib/s3.go	0% <0%> (ø)	⬆️
lib/session.go	0% <0%> (ø)
lib/config.go	92.85% <100%> (+0.26%)	⬆️
lib/store.go	74.22% <52.63%> (-5.78%)	⬇️
lib/files.go	81.44% <60%> (-1.72%)	⬇️
lib/cloudfront.go	68.68% <68.68%> (ø)
lib/deployer.go	81.32% <77.77%> (+0.43%)	⬆️

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