Normalize paths in deprecation messages

[sambostock]

Summary

This (tentatively) adds support for registering deprecation message normalizers, and provides built in normalization of paths by looking for the following strings:

• appearances of Dir.pwd, which are replaced with #{Dir.pwd}
• appearances of any of Gem.path's paths, which are replaced with #{Gem.path[index]}, where index is the index of the path in the Gem.path array

Replacements are made to look like interpolation, so it is clear what they represent.

This should resolve #48.

⚠️ Before Merging

• Validate approach
  • Should normalizers be expected to mutate the message (to avoid duplicate string allocations), or is it clearer if they return the normalized message (at the cost of creating duplicate objects)?
• Write tests
• Consider other safe built in replacements that may have been missed.
  • What happens if a gem is sourced from somewhere non standard? e.g. GitHub? Path?
• Document new config in README.md
• Update CHANGELOG.md

[iangreenleaf]

@Edouard-chin is there any interest in incorporating something like this PR? I have run into a different problem that was solvable with this same concept. In my case, we have a deprecation warning that includes a whole hash object in the message, making it unstable between test runs. I needed to transform the message before persisting, to "redact" the variable part so it can be logged as a generic message:

warning: detected duplicate key "event_type" in [HASH]. This will raise an error in json 3.0 unless enabled via allow_duplicate_key: true

I used a slightly more generic idea of "transformations" on the event payload that adds a little more flexibility, but I could have accomplished this with the normalizers in the original PR as well:

module DeprecationToolkit
  class Configuration
    config_accessor(:transformations) { [] }
  end

  class DeprecationSubscriber < ActiveSupport::Subscriber
    def deprecation(event)
      event.payload = Configuration
        .transformations
        .reduce(event.payload) { |payload, transformation| transformation.call(payload) }

      super(event)
    end
  end
end

DeprecationToolkit::Configuration.transformations = [
  ->(payload) {
    re = /(.*json\/encoding\.rb.*warning: detected duplicate key "\w+") in .*\.\s*(This will raise an error.*)/
    re.match(payload[:message]) do |matchdata|
      payload[:message] = "#{matchdata[1]}. #{matchdata[2]}"
    end

    payload
  }
]