V2 Implement NodeCache

[kyle-ssg]

#16 allowed the ability to manage a cache of flags and users.

How it worked:

1. When initialising the SDK with a cache parameter, it first validated the values here

2. When retrieving a flag, it will query async cache.has("flags"), if it returns a value it will skip hitting the APIhere

3. When flags have been retrieved it will set the cache with cache.set("flags") here

4. When retrieving a flag for an identity, query async cache.has("identity_id"), if it returns a value it will skip hitting the API here

5. When flags have been retrieved for an identity set the cache with cache.set("identity_id") here

[eder-zadrima]

@kyle-ssg , I've added this into the v2 branch here, feel free to take a look.