Add support for silencing backtraces

[jcreixell]

Rails backtraces can be very verbose. This not only leads to noisy traces where it is difficult to find relevant information, but it also interferes with how effectively periskop is able to group exceptions (since it uses a maximum number of backtrace lines for efficiency reasons, see

periskop-ruby/lib/periskop/client/models.rb

Line 104 in 03e17f1

stacktrace_head = @exception_instance.stacktrace.first(MAX_TRACES).join('')

).

Rails introduced the concept of BacktraceCleaner a while ago, we should probably implement something similar, potentially configurable with some well known instances ready to use. See https://www.rubydoc.info/gems/activesupport/2.3.5/ActiveSupport/BacktraceCleaner for inspiration.