Add a load hook for ActiveModel::Model

[lewispb]

Motivation / Background

ActiveRecord::Base has a dedicated ActiveSupport load hook. This adds a hook for ActiveModel::Model, so that when ActiveModel is being used without ActiveRecord, it can still be modified.

Kredis would be able to use this hook here. Instead of:

config.after_initialize do
  ActiveModel::Model.include Kredis::Attributes if defined?(ActiveModel::Model)
end

we could write

ActiveSupport.on_load(:active_model) do
  include Kredis::Attributes
end

This change would help fix a reported issue at rails/kredis#60.

Detail

See the motivating issue at rails/kredis#60 and additional comment by @fxn here: rails/kredis#60 (comment)

Checklist

Before submitting the PR make sure the following are checked:

• This Pull Request is related to one change. Changes that are unrelated should be opened in separate PRs.
• Commit message has a detailed description of what changed and why. If this PR fixes a related issue include it in the commit message. Ex: [Fix #issue-number]
• Tests are added or updated if you fix a bug or add a feature.
• CHANGELOG files are updated for the changed libraries if there is a behavior change or additional feature. Minor bug fixes and documentation changes should not be included.