Add beforeLint option

This allows a user to perform operations on the imported CSS before
passing it along to be linted. An example might be to run
`postcss-nested` first.

Author: simonsmith

lib/index.js

@@ -98,7 +98,15 @@ function mergeOptions(options) {
   // Set some core options
   merged.minify = options.minify;
   merged['postcss-import'].root = options.root;
-  merged['postcss-import'].transform = lintImportedFiles(merged);
+
+  // Call beforeLint function and pass processed css to bem-linter
+  var beforeLint = options.beforeLint;
+  merged['postcss-import'].transform = function(css, filename) {
+    if (typeof beforeLint === 'function') {
+      css = beforeLint(css, filename, merged);
+    }
+    return lintImportedFiles(merged)(css, filename);
+  };
 
   // Allow additional plugins to be merged with the defaults
   // but remove any duplicates so that it respects the new order

package.json

@@ -34,7 +34,8 @@
     "chai": "^3.4.1",
     "mocha": "^2.3.4",
     "postcss-property-lookup": "^1.1.4",
-    "rewire": "^2.5.0"
+    "rewire": "^2.5.0",
+    "sinon": "^1.17.2"
   },
   "scripts": {
     "test": "mocha --reporter spec --slow 400",

test/test.js

@@ -1,4 +1,5 @@
 var expect = require('chai').expect;
+var sinon = require('sinon');
 var child = require('child_process');
 var exec = child.exec;
 var spawn = child.spawn;
@@ -114,6 +115,47 @@ describe('suitcss', function () {
         ]);
       });
     });
+
+    describe('beforeLint option', function() {
+      var lintImportedFilesStub, bemLintStub, beforeLintStub, revert;
+
+      beforeEach(function() {
+        lintImportedFilesStub = sinon.stub();
+        bemLintStub = sinon.stub().returns('/* lint */');
+        beforeLintStub = sinon.stub().returns('/* before lint */');
+
+        lintImportedFilesStub.onFirstCall().returns(bemLintStub);
+        revert = suitcss.__set__('lintImportedFiles', lintImportedFilesStub);
+      });
+
+      afterEach(function() {
+        revert();
+      });
+
+      it('should call user supplied function before linting', function (done) {
+        suitcss(read('fixtures/component'), {
+          root: 'test/fixtures',
+          beforeLint: beforeLintStub
+        }).catch(done);
+
+        expect(bemLintStub.calledOnce).to.be.ok;
+        expect(beforeLintStub.calledOnce).to.be.ok;
+        expect(beforeLintStub.calledBefore(bemLintStub)).to.be.ok;
+
+        done();
+      });
+
+      it('should pass processed CSS to the linting transform function', function (done) {
+        suitcss(read('fixtures/component'), {
+          root: 'test/fixtures',
+          beforeLint: beforeLintStub
+        }).catch(done);
+
+        expect(bemLintStub.getCall(0).args[0]).to.equal('/* before lint */');
+
+        done();
+      });
+    });
   });
 });