inline util._extend, rather than requiring util

Related:
- #25
- #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.

git-host.js

@@ -1,8 +1,21 @@
 'use strict'
 var gitHosts = require('./git-host-info.js')
 /* eslint-disable node/no-deprecated-api */
+
+// copy-pasta util._extend from node's source, to avoid pulling
+// the whole util module into peoples' webpack bundles.
 /* istanbul ignore next */
-var extend = Object.assign || require('util')._extend
+var extend = Object.assign || function _extend (target, source) {
+  // Don't do anything if source isn't an object
+  if (source === null || typeof source !== 'object') return target
+
+  const keys = Object.keys(source)
+  let i = keys.length
+  while (i--) {
+    target[keys[i]] = source[keys[i]]
+  }
+  return target
+}
 
 var GitHost = module.exports = function (type, user, auth, project, committish, defaultRepresentation, opts) {
   var gitHostInfo = this