Skip zap stack frames in stacktrace output

[prashantv]

Currently, zap only filters out runtime stackframes, which leaves a lot
of noise from zap (Error, log, check, etc). This modifies the stacktrace
to filter out all zap frames at the beginning of the stack. It will not
filter out zap stacks in between non-zap frames.

Fixes #488.

[codecov]

Codecov Report

Merging #491 into master will increase coverage by 0.03%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #491      +/-   ##
==========================================
+ Coverage    96.9%   96.93%   +0.03%     
==========================================
  Files          36       36              
  Lines        2197     2219      +22     
==========================================
+ Hits         2129     2151      +22     
  Misses         59       59              
  Partials        9        9

Impacted Files	Coverage Δ
stacktrace.go	90.47% <100%> (+5.11%)	⬆️

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[akshayjshah]

Looks great.