Added WithPanicPropagation function

[yklyahin]

I added a feature for panic propagation.

More details:
#540
https://discuss.elastic.co/t/go-agent-apmhttp-middleware/193927/2

[axw]

Thanks @lobotomist, looks great!

[axw]

jenkins run the tests please

[codecov-io]

Codecov Report

Merging #611 into master will increase coverage by 0.05%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #611      +/-   ##
==========================================
+ Coverage   87.74%   87.79%   +0.05%     
==========================================
  Files         122      122              
  Lines        7476     7483       +7     
==========================================
+ Hits         6560     6570      +10     
+ Misses        814      810       -4     
- Partials      102      103       +1

Impacted Files	Coverage Δ
module/apmhttp/handler.go	83.2% <100%> (-0.7%)	⬇️
tracer.go	89.69% <0%> (+0.32%)	⬆️
transport/http.go	95.41% <0%> (+1.14%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update bfee654...12143d4. Read the comment docs.