Add post response hook

[kolanos]

Closes #338

[codecov]

Codecov Report

Merging #339 into master will increase coverage by 0.07%.
The diff coverage is 94.11%.

@@            Coverage Diff             @@
##           master     #339      +/-   ##
==========================================
+ Coverage   83.55%   83.62%   +0.07%     
==========================================
  Files          36       36              
  Lines        1368     1380      +12     
==========================================
+ Hits         1143     1154      +11     
- Misses        225      226       +1

Impacted Files	Coverage Δ
iopipe/contrib/eventinfo/plugin.py	100% <100%> (ø)	⬆️
iopipe/agent.py	87.67% <100%> (+0.17%)	⬆️
iopipe/contrib/logger/plugin.py	97.61% <100%> (+0.05%)	⬆️
iopipe/contrib/profiler/plugin.py	96.55% <100%> (+0.12%)	⬆️
iopipe/contrib/trace/plugin.py	100% <100%> (ø)	⬆️
iopipe/plugins.py	63.88% <50%> (-0.82%)	⬇️

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