Add stability indicator

[hunvreus]

Add support for a message informing the reader of the stability of each method.

[hunvreus]

Only reviewing this now. @xufeng123 : why do we have that many codes? 5 or 6? What are they? Can't we simply have 3? Something like: alpha, beta and stable.

[fraserxu]

Because the official node document has this. I just copy that from there.

[fraserxu]

Whenever we need that in the docs, just add status:'stab05', something like that

---
title: 'Response status codes'
status: 'stab05'

layout: nil
---

And then it will be added automatically to the site.

[vivi90]

@fraserxu Hi, cant find this indices (stab05).