module/apmhttp: support ELASTIC_APM_IGNORE_URLS

[axw]

Add support for ignoring requests with URLs matching the
regular expression specified in ELASTIC_APM_IGNORE_URLS.
Ignore requests are executed as normal, they just won't
be reported to Elastic APM.

Closes #154

[codecov-io]

Codecov Report

Merging #158 into master will decrease coverage by 0.41%.
The diff coverage is 45.9%.

@@            Coverage Diff             @@
##           master     #158      +/-   ##
==========================================
- Coverage   77.54%   77.12%   -0.42%     
==========================================
  Files          69       70       +1     
  Lines        3945     3992      +47     
==========================================
+ Hits         3059     3079      +20     
- Misses        668      696      +28     
+ Partials      218      217       -1

Impacted Files	Coverage Δ
module/apmhttp/client.go	73.33% <100%> (ø)	⬆️
module/apmhttprouter/handler.go	63.41% <25%> (-8.02%)	⬇️
module/apmgin/middleware.go	67.12% <40%> (-3.65%)	⬇️
module/apmgorilla/middleware.go	73.33% <44.44%> (-13.04%)	⬇️
module/apmhttp/ignorer.go	47.05% <47.05%> (ø)
module/apmhttp/handler.go	66.07% <50%> (-0.6%)	⬇️
module/apmecho/middleware.go	85.24% <57.14%> (-6.92%)	⬇️
tracer.go	84.92% <0%> (+0.57%)	⬆️

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