Split localStorage and sessionStorage

[ivan-kleshnin]

Wouldn't it be better to split two targets onto two different drivers?

1. I don't think that amount of if checks (we have now in the code) is better than possible code duplication.
2. sessionStorage and localStorage have different level of support among browsers.
   So library can work with one and not work with another. Changing driver factory implies this more than
   changing key.
3. "Controlling argument" is kinda of bad programming style.
4. CycleJS driver is about side effect type. LocalStorage and SessionStorage are two different targets. They may have considerably different implementation, performance characteristics, etc.
   Two drivers better describe what goes on in reality.

[kahlil]

Since I implemented this driver per @staltz's requirements and I am fine with how it works I would ask you to discuss this directly with him.

[jvanbruegge]

My storage driver (cyclejs-storage) supports both, seperated use and use as single driver