Provide API to allow users to add their own modules.

[rinatkhaziev]

It'd be nice to provide API for users to add their own Edit Flow modules for things like integrations with other plugins (See #386 and #387 as examples).

ef_register_module(...)

Details of the implementation are yet to be discussed.

[WPprodigy]

This would be a good time to separate out the module loading process. There could be a separate class responsible for holding all the instances of modules (with register/unregister/get methods).

Then the base Edit Flow class would add its core modules, and expose the method for registering new ones. On the init hook, it'd run through the list and call $module->init(); on those that are enabled.