Don't mutate global config object

Closes #152

index.js

@@ -24,7 +24,7 @@ const output = argv.output
 
 if (argv.map) argv.map = { inline: false }
 
-let config = {
+const cliConfig = {
   options: {
     map: argv.map !== undefined ? argv.map : { inline: true },
     parser: argv.parser ? require(argv.parser) : undefined,
@@ -117,7 +117,7 @@ Promise.resolve()
   })
 
 function rc(ctx, path) {
-  if (argv.use) return Promise.resolve()
+  if (argv.use) return Promise.resolve(cliConfig)
 
   // Set argv: false to keep cosmiconfig from attempting to read the --config
   // flag from process.argv
@@ -128,7 +128,7 @@ function rc(ctx, path) {
           'Config Error: Can not set from or to options in config file, use CLI arguments instead'
         )
       }
-      config = rc
+      return rc
     })
     .catch(err => {
       if (err.message.indexOf('No PostCSS Config found') === -1) throw err
@@ -175,7 +175,7 @@ function css(css, file) {
   printVerbose(chalk`{cyan Processing {bold ${relativePath}}...}`)
 
   return rc(ctx, argv.config)
-    .then(() => {
+    .then(config => {
       const options = config.options
 
       if (file === 'stdin' && output) file = output