add capability to pass other options as function

syntax, parser, and stringifier can now be returned from an object
in options.postcss and therefore passed as function, not strings
only.

Author: Jeff Escalante

README.md

@@ -198,3 +198,25 @@ var css = require('postcss?parser=postcss-safe-parser!./broken')
 ```
 
 [Safe Parser]: https://github.com/postcss/postcss-safe-parser
+
+If you need to pass the function directly instead of a module name, you can do so through the webpack postcss option, as such:
+
+```js
+var sugarss = require('sugarss')
+module.exports = {
+    module: {
+        loaders: [
+            {
+                test:   /\.css$/,
+                loader: "style-loader!css-loader!postcss-loader"
+            }
+        ]
+    },
+    postcss: function () {
+        return {
+            plugins: [autoprefixer, precss],
+            syntax: sugarss
+        };
+    }
+}
+```

index.js

@@ -19,21 +19,31 @@ module.exports = function (source, map) {
     if ( typeof map === 'string' ) map = JSON.parse(map);
     if ( map && map.mappings ) opts.map.prev = map;
 
-    if ( params.syntax )      opts.syntax      = require(params.syntax);
-    if ( params.parser )      opts.parser      = require(params.parser);
-    if ( params.stringifier ) opts.stringifier = require(params.stringifier);
-
-    var plugins = this.options.postcss;
-    if ( typeof plugins === 'function' ) {
-        plugins = plugins.call(this, this);
+    var plugins;
+    var options = this.options.postcss;
+    if ( typeof options === 'function' ) {
+        options = options.call(this, this);
     }
 
-    if ( typeof plugins === 'undefined' ) {
+    if ( typeof options === 'undefined' ) {
         plugins = [];
     } else if ( params.pack ) {
-        plugins = plugins[params.pack];
-    } else if ( !Array.isArray(plugins) ) {
-        plugins = plugins.defaults;
+        plugins = options[params.pack];
+    } else if ( !Array.isArray(options) ) {
+        plugins = options.plugins || options.defaults;
+        opts.syntax = options.syntax;
+        opts.parser = options.parser;
+        opts.stringifier = options.stringifier;
+    }
+
+    if ( params.syntax && !opts.syntax ) {
+        opts.syntax = require(params.syntax);
+    }
+    if ( params.parser && !opts.parser ) {
+        opts.parser = require(params.parser);
+    }
+    if ( params.stringifier && !opts.stringifier ) {
+        opts.stringifier = require(params.stringifier);
     }
 
     var loader   = this;