Allow encapsulating all ls items under a prefix to avoid clearing items from different parts of the application

[adamtal3]

I've integrated localstorage-slim into an existing application and noticed that clearing clears everything in localStorage (even if created directly with localStorage and not localstorage-slim).

Adding a key prefix value in the global configuration could help create an encapsulated storage inside localStorage without affecting other items.

[adamtal3]

PR #24