Allow options to be passed to postcss

Fixes #22

Author: simonsmith

README.md

@@ -153,6 +153,20 @@ Third paramater is the options object containing any PostCSS configuration you m
 }
 ```
 
+##### `postcss`
+
+* Type: `Object`
+* Default: `undefined`
+
+Options that are passed directly to `postcss`, as per [the documentation](https://github.com/postcss/postcss/blob/master/docs/api.md#processorprocesscss-opts).
+
+```js
+{
+  postcss: {from: 'filename.css'}
+}
+```
+
+
 ##### `use`
 
 * Type: `Array`

lib/index.js

@@ -75,7 +75,7 @@ function preprocessor(css, options) {
     processor.use(cssnano(options.cssnano));
   }
 
-  return processor.process(css);
+  return processor.process(css, options.postcss);
 }
 
 /**

test/test.js

@@ -80,6 +80,37 @@ describe('suitcss', function() {
       expect(opts['postcss-import'].root).to.equal('test/root');
     });
 
+    describe('passing options to postcss', function() {
+      var postcssStub, processMethodStub, revert;
+
+      beforeEach(function() {
+        postcssStub = sinon.stub();
+        processMethodStub = sinon.stub();
+
+        postcssStub.returns({
+          use: sinon.spy(),
+          process: processMethodStub
+        });
+        revert = suitcss.__set__('postcss', postcssStub);
+        suitcss('body {}', {
+          root: 'something',
+          postcss: {
+            from: 'somefile.css'
+          }
+        });
+      });
+
+      afterEach(function() {
+        revert();
+      });
+
+      it('should pass postcss options to the processor', function() {
+        expect(processMethodStub.getCall(0).args[1]).to.eql({
+          from: 'somefile.css'
+        });
+      });
+    });
+
     describe('re-ordering plugins', function() {
       it('should allow reordering of use array and remove duplicates', function() {
         var opts = mergeOptions({