Return all item's keys, doen't remove expired items
keys()All item's keys (array)
OhMyCache.Local.set('k1', 'v1')
OhMyCache.Local.set('k2', 'v2', {readonly: true})
OhMyCache.Local.set('k3', 'v3', {expire: 1})
// sleep 2 secondes or more
OhMyCache.Local.keys() // ['k1', 'k2', 'k3']