Add support for custom authorizers with SAM

[kyleknap]

This now works for the chalice package command and scopes configuration and policies to the specifically configured lambda functions.

[codecov-io]

Codecov Report

Merging #580 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #580      +/-   ##
==========================================
+ Coverage   94.65%   94.68%   +0.02%     
==========================================
  Files          18       18              
  Lines        3164     3179      +15     
  Branches      412      413       +1     
==========================================
+ Hits         2995     3010      +15     
  Misses        118      118              
  Partials       51       51

Impacted Files	Coverage Δ
chalice/deploy/swagger.py	100% <100%> (ø)	⬆️
chalice/package.py	97.67% <100%> (ø)	⬆️
chalice/utils.py	96.47% <100%> (+0.12%)	⬆️

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