Allow users to configure their asset-skipping pattern

[st0012]

Some users have customized asset paths, so the SDK's default pattern may not work for them. In such cases, they can use the new configuration option to set their own path pattern.

Sentry.init do |config|
  config.rails.assets_regexp = /my_regexp/
end

[codecov-commenter]

Codecov Report

Base: 98.45% // Head: 98.45% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (4408ee1) compared to base (b54ba68).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1915   +/-   ##
=======================================
  Coverage   98.45%   98.45%           
=======================================
  Files         151      151           
  Lines        9259     9279   +20     
=======================================
+ Hits         9116     9136   +20     
  Misses        143      143           

Impacted Files	Coverage Δ
...entry-rails/lib/sentry/rails/capture_exceptions.rb	100.00% <100.00%> (ø)
sentry-rails/lib/sentry/rails/configuration.rb	100.00% <100.00%> (ø)
sentry-rails/spec/sentry/rails/tracing_spec.rb	99.36% <100.00%> (+0.07%)	⬆️

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[st0012]

@sl0thentr0py I'll add a changelog before merging it