Add stupid benchmarks per feature

Ref #65

First shot as today gives

```
❯ node benchmarks.js
customProperties takes 0.20704s
calc takes 0.03314s
customMedia takes 0.01792s
mediaQueriesRange takes 0.00871s
customSelectors takes 0.00889s
colorRebeccapurple takes 0.07094s
colorHwb takes 0.07437s
colorGray takes 0.09796s
colorHexAlpha takes 0.07138s
colorFunction takes 0.11102s
fontVariant takes 0.00786s
filter takes 0.08778s
rem takes 0.01469s
autoprefixer takes 1.04424s
``

Author: MoOx

benchmarks.js

@@ -0,0 +1,23 @@
+var cssnext = require("./")
+var t = require("microtime")
+var assign = require("object-assign")
+
+var input = require("fs").readFileSync("./test/fixtures/cases/example.css")
+
+// test each features
+var keys = Object.keys(cssnext.features)
+var allOff = {}
+
+keys.forEach(function(k) {
+  allOff[k] = false
+})
+
+keys.forEach(function(k) {
+  var enable = assign({}, allOff)
+  enable[k] = true
+  var start = t.now()
+  cssnext(input, {features: enable})
+  var stop = t.now()
+
+  console.log(k + " takes " + ((stop - start) / 100000) + "s") //, enable)
+})

package.json

@@ -60,6 +60,7 @@
     "browserify": "^8.1.1",
     "jscs": "^1.6.2",
     "jshint": "^2.5.6",
+    "microtime": "^1.2.0",
     "tape": "^3.0.0"
   },
   "scripts": {