Return array of keys in storage

I was looking to use this on another project and I needed to get the list of keys in storage. This seemed like a core piece of functionality rather than writing a plugin for it.
A note here: accessing `Basil.options.namespace` is not safe here because the namespace could have been overridden while instantiating Basil (given as parameters [here](https://github.com/Wisembly/basil.js/pull/24/files?diff=unified#diff-960dfaa4b436d848d7b2bf4e3378564eL185)).

More globally, I think storages' `key` methods should be totally Basil namespace agnostic. There are just here as storage interface and allow Basil to have its unified interface. Basil namespace should only live in Basil public `key` API. This `key` method should "just" return all localStorage keys. Same for session or cookie.