inline util._extend, rather than requiring util
inline util._extend, rather than requiring util

Related:
- https://github.com/npm/hosted-git-info/pull/25
- https://github.com/npm/hosted-git-info/pull/40

This removes the require('util') fallback for Object.assign, without
dropping support for node v4 (which will happen on the next major,
likely along with 6), but gets bundle sizes down in the meantime.

npm/hosted-git-info