Reorganize module options

[shioyama]

Currently there is a lot of hard-coded dependencies between options-setting in
the Mobility::Attributes class and the various backend modules (cache,
fallbacks, etc.), whereas these should really be independent.

I'm going to try to restructure things so that modules can be defined in such a
way that Mobility::Attributes does not need to explicitly know anything about
them when including them from options.

[shioyama]

This change adds extracts hard-coded dependencies in the Mobility::Attributes class to the different "option modules" (cache, fallbacks, etc) so that the order and set of modules to include can be entirely customized.

Two new configuration options can be used to do this:

Mobility.option_modules         
#=> {
#   :cache => Mobility::Backend::Cache,          
#   :dirty => Mobility::Backend::Dirty,             
#   :fallbacks => Mobility::Backend::Fallbacks,     
#   :presence => Mobility::Backend::Presence,       
#   :default => Mobility::Backend::Default,         
#   :fallthrough_accessors => Mobility::FallthroughAccessors,                                      
#   :locale_accessors => Mobility::LocaleAccessors
# }

This is a hash of option keys and constants (classes or modules). The only requirement of the classes used as hash values is that they have a singleton method apply which takes an instance if Attributes and an option value (typically a boolean) and then does something with the Attributes instance, typically including a module into attributes.backend_class or into attributes itself.

This hash is iterated over here when initializing an Attributes module, taking the option value for the key and passing it and the attributes module itself to the option module (e.g. Mobility::Backend::Class.apply(attributes, true), if cache: true was passed to translates).

Mobility.default_options
#=> {:cache=>true, :dirty=>false, :fallbacks=>nil, :presence=>true, :default=>nil}

The other configuration option is Mobility.default_options. This gets merged into the options passed into translates, thus allowing you to set a hash of defaults to be used across all your models.

[valscion]

Thank you for creating these detailed pull requests, and elaborating your thoughts here! Even though it might feel like no-one is reading these, I for one am grateful to be able to follow along how this gem evolves, and how you think through these changes.

🙇

[shioyama]

Thanks! That's great to hear. I was mainly doing these PRs as mental notes to myself, with the idea that anybody who was interested could follow along, but very glad to hear that they are useful to you and others. Really appreciate that you took the time to express that here 😄