api: Add support for GET /realm/filters.

[Anupam-dagar]

Issue #63
@aero31aero I found out that the emoji.js file present already in the repository does the same which this PR does. I am not sure to remove that and keep this or remove this and keep that.
IMO removing that will be good as there is one more endpoint to implement for realm.

Update: This PR consist of 2 commits as follows:

1. This commit add endpoint to get realm emojis.
2. This commit add endpoint to get realm filters.

Testing:
Working of the endpoints has been tested by node tests and by doing a real time query on a server.

[aero31aero]

@arpith since you are probably already using the emoji feature, what is your take on this?

I feel that having z.filters.retrieve() (new) and z.emojis.retrieve() (already exists) should be better than keeping them under a realm. I'd prefer abstracting out the realm detail from the API as that's just our internal implementation. Anyone dealing with a single API key is dealing with a single realm at a time.

[arpith]

Yeah I agree!

[aero31aero]

@Anupam-dagar can you rebase this and follow the suggestion in my previous comment?

[Anupam-dagar]

@aero31aero please review