📝 docs(gitbook): more more tweaks

👕 refactor(flowtype): update flowtype definitions

✨ release(patch): v6.0.0-alpha.3

✨ release(patch): v6.0.0-alpha.3

👕 refactor(decls): add vue types

📦 build(gitbook): update docs

👕 refactor: tweak util

📝 docs(example): add custom formatter example

⚡ improvement(npm): revert node >= 6.0 engine restriction (kazupon#110)…

… by @syxolk

* 🔙 config(npm): revert node >= 6.0 engine restriction

Partially revert commit a2ec64f that introduced
backwards-incompatible change in 5.0.1.

Fixes kazupon#109

* 💚 test: add 'use strict' to fix tests for node 4

📝 docs(README): update circleci badge