Upgrading to node v4/v5 version

Hello @defunctzombie. I was just wondering whether you considered upgrading the utils to match the new features of the `util` module introduced by node >=4. Specifically ES2015 data types support for `util.inspect`? Would you accept a PR concerning this issue?

No file or test is named. Start by reviewing the repository's util implementation and comparing its inspect behavior with the Node.js v4/v5 util module, then define the ES2015 data types that need support and add coverage before proposing a PR.

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