Support creation of custom store modules

[christhekeele]

Currently there's the hidden Mnemonix.Singleton that kind of does this.

Preferably you'd be able to insert the entire Mnemonix DSL into a module, in addition to the singleton variant. Something like Mnemonix.API and Mnemonix.API.Singleton.

Ideally start_link would be overridable and the implementation would do more than just defer to Mnemonix, but instead make the direct GenServer calls. So the Mnemonix module would eat our own dogfood here and simply use Mnemonix.API.

This would mostly be hiding and extending the Mnemonix API inside a library offering like Phoenix.CacheStore or something that could add extra use-case specific functions.

Needs to be determined how this would play with #9 if both were implemented.

[christhekeele]

Mnemonix.API is implemented but not documented. Mnemonix.Singleton is outdated and undocumented.

[christhekeele]

Closed in v0.6.0.