Add more labels for triage

Inspired by [Servo's naming rules for labels](https://github.com/servo/servo/wiki/Tag-label-names-and-definitions), I've added `A-audit` which `A` means `Area` and also `B-meta` for meta issues (`B` is for categorizing in Servo though :P).

I think it would be great to add more labels for easier triage and identity.

cc @wusyong @weihanglo 