Add session storage support with function option

[ekwoka]

Great work dude. I've never done a pull request before, so please let me know if I did anything wrong in this process, just edited the file on github. Worked on a separate thing I was playing with. Seems this could be the cleanest way to add session storage support without breaking anything for anyone else and keeping it DRY.

[ekwoka]

forgot to write how to use:

Alpine.persistedStore('this is in', 'localStorage');
Alpine.persistedStore('this is in', 'sessionStorage', sessionStorage);

[ryangjchandler]

@ekwoka Can you confirm that these changes work?

[ekwoka]

@ekwoka Can you confirm that these changes work?

@ryangjchandler This works brilliantly

[ryangjchandler]

@ekwoka Could you also update the documentation?

[ekwoka]

@ryangjchandler done. Should I do the build as well? Is that normal?

[ryangjchandler]

Yes please, if you could build and push those files.

[ekwoka]

@ryangjchandler

Done. I guess you didn't see that I built it a while ago haha