Properly verify webhook signature

[michaelbeaumont]

Closes #6

[codecov-io]

Codecov Report

Merging #10 into master will decrease coverage by 1.9%.
The diff coverage is 95.65%.

@@            Coverage Diff             @@
##           master      #10      +/-   ##
==========================================
- Coverage   89.65%   87.75%   -1.91%     
==========================================
  Files           3        3              
  Lines          29       49      +20     
  Branches        8       12       +4     
==========================================
+ Hits           26       43      +17     
- Misses          3        6       +3

Impacted Files	Coverage Δ
index.js	86.95% <95.65%> (-1.51%)	⬇️

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