Ignore css output if it's not a string

Fix thgh#26
Resolve thgh#37

index.es.js

@@ -90,6 +90,10 @@ export default function css (options = {}) {
           return
         }
 
+        if (typeof css !== 'string') {
+          return
+        }
+
         if (typeof dest !== 'string') {
           // Don't create unwanted empty stylesheets
           if (!css.length) {
@@ -134,8 +138,8 @@ function getSize (bytes) {
   return bytes < 10000
     ? bytes.toFixed(0) + ' B'
     : bytes < 1024000
-    ? (bytes / 1024).toPrecision(3) + ' kB'
-    : (bytes / 1024 / 1024).toPrecision(4) + ' MB'
+      ? (bytes / 1024).toPrecision(3) + ' kB'
+      : (bytes / 1024 / 1024).toPrecision(4) + ' MB'
 }
 
 function ensureParentDirsSync (dir) {

package.json

@@ -9,6 +9,7 @@
     "build": "rollup -c",
     "dev": "rollup -cw",
     "lint": "standard rollup.config.js index.es.js",
+    "fix": "standard --fix rollup.config.js index.es.js",
     "test": "cd test && rm -f output.* && rollup -c && cmp output.js expected.js && cmp output.css expected.css && cd ..",
     "prepare": "rollup -c"
   },
@@ -39,6 +40,6 @@
   "devDependencies": {
     "rollup": "1",
     "rollup-plugin-buble": "0",
-    "standard": "13"
+    "standard": "14"
   }
 }

rollup.config.js

@@ -4,9 +4,10 @@ export default {
   input: 'index.es.js',
   output: {
     file: 'index.cjs.js',
-    format: 'cjs',
+    format: 'cjs'
   },
   plugins: [
     buble()
-  ]
+  ],
+  external: ['fs', 'path', 'rollup-pluginutils']
 }