Replace deprecated gulp-util

Closes GH-19
Reviewed-by: Christian Murphy <christian.murphy.42@gmail.com>

index.js

@@ -3,7 +3,7 @@
 var PassThrough = require('stream').PassThrough
 var engine = require('unified-engine')
 var xtend = require('xtend')
-var PluginError = require('gulp-util').PluginError
+var PluginError = require('plugin-error')
 var through = require('through2')
 var convert = require('convert-vinyl-to-vfile')
 

package.json

@@ -19,9 +19,10 @@
   ],
   "dependencies": {
     "convert-vinyl-to-vfile": "^2.0.0",
-    "gulp-util": "^3.0.7",
+    "plugin-error": "^1.0.1",
     "through2": "^2.0.1",
     "unified-engine": "^6.0.0",
+    "vinyl": "^2.2.0",
     "xtend": "^4.0.1"
   },
   "devDependencies": {

test/index.js

@@ -1,16 +1,14 @@
 'use strict'
 
 var PassThrough = require('stream').PassThrough
-var util = require('gulp-util')
+var PluginError = require('plugin-error')
+var File = require('vinyl')
 var test = require('tape')
 var html = require('remark-html')
 var engine = require('..')
 var example = require('./example')
 var spy = require('./spy')
 
-var File = util.File
-var PluginError = util.PluginError
-
 var input = ['# h1', '', '', '## h2', ''].join('\n')
 
 var report = [