Load config/appsignal.rb by default

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To do

• Load config/appsignal.rb if present on Appsignal.start.
  • I don't think we should add this logic to the Config class itself, but lib/appsignal.rb instead on start.
  • It should also not be loaded if an app calls Appsignal.configure somewhere in the app, like a Rails initializer.
• Ignore the config/appsignal.yml YAML file if the .rb variant is present.
• Load the config/appsignal.rb file automatically in the diagnose CLI. Hopefully it would only be necessary to add test for it.
• Make sure it works well in combination with Appsignal.configure being called in a Rails initializer. It will either listen to Appsignal.configure or load the config/appsignal.rb file.
• Add config option to the installer for an config/appsignal.rb file.
  • Add default config that sort of mimics the YAML file, but figure out the best way to disable for the test env etc.
• Make the config option for the .rb the recommended option in the installer.

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PR in #1324

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PR for the installer in #1327