Enable Rails/ActiveSupport backtrace cleaner in config

[waltjones]

Fixes: #900

Allows setting a Rails or ActiveSupport backtrace cleaner in the config.

Example:

Rollbar.configure do |config|
  config.backtrace_cleaner = Rails::BacktraceCleaner.new
end

This option is compatible with any object that implements the ActiveSupport::BacktraceCleaner interface.

It is also possible to apply a backtrace cleaner in a before_process handler, but it will have no effect on traces generated using populate_empty_backtraces. The new option is compatible with populate_empty_backtraces, is simpler to use, and will be called only for message types that contain a stack trace.