chore(CHANGELOG): introduce perf() section for performance-related commits

Instead of using fix() or chore() when labelling a commit which improves
speed or performance use perf(). Perf commits will be listed in the
CHANGELOG under "Performance Improvements".

For example:
perf($animate): cache all getComputedStyle operations to reduce additional reflows

validate-commit-msg.js

@@ -21,6 +21,7 @@ var TYPES = {
   docs: true,
   style: true,
   refactor: true,
+  perf: true,
   test: true,
   chore: true,
   revert: true