Add Performance category

There are classes of issues that are more precisely described as
having a potential negative performance impact. Recent examples from
engines tuning include eslint's no-extra-bind
(http://eslint.org/docs/rules/no-extra-bind) and csslint's performance
category (https://github.com/CSSLint/csslint/wiki/rules#performance).

Author: jpignata

SPEC.md

@@ -96,6 +96,7 @@ Issues must be associated with one or more categories. Valid issue `categories`
 - `Compatibility` -- TODO describe me
 - `Complexity` -- TODO describe me
 - `Duplication` -- TODO describe me
+- `Performance` -- TODO describe me
 - `Security` -- TODO describe me
 - `Style` -- TODO describe me