Add context as iopipe instance attribute

[kolanos]

Closes #327

[codecov]

Codecov Report

Merging #328 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #328      +/-   ##
==========================================
+ Coverage    83.5%   83.51%   +0.01%     
==========================================
  Files          36       36              
  Lines        1346     1347       +1     
==========================================
+ Hits         1124     1125       +1     
  Misses        222      222

Impacted Files	Coverage Δ
iopipe/agent.py	88.57% <100%> (+0.08%)	⬆️

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[mrickard]

Cool!