suggest add sessionStorage

[kaimzhou]

Because sometimes sessionStorage is needed, so this is my suggest.
just like this
Vue.use(VueLocalStorage, {
localStorageName: 'ls',
sessionStorageName: 'ss',
bind: true
})

[Fohlen]

I would like to work on this feature!
Ideally one would create a storage-unagnostic system where you could switch between multiple implementations of local/sessionstorage as long as they are standards compliant!