Add addModulesDirectories config (#256)

* Add `addModulesDirectories` config

Allows appending to the default list of modules directories.

Relevant discussion:

#195

Author: neezer

README.md

@@ -198,6 +198,19 @@ postcss: function(webpack) {
 }
 ```
 
+#### `addModulesDirectories`
+
+Type: `Array`  
+Default: `[]`
+
+An array of folder names to add to [Node's resolver](https://github.com/substack/node-resolve).
+Values will be appended to the default resolve directories: 
+`["node_modules", "web_modules"]`.
+
+This option is only for adding additional directories to default resolver. If
+you provide your own resolver via the `resolve` configuration option above, then
+this value will be ignored.
+
 #### Example with some options
 
 ```js

index.js

@@ -16,6 +16,7 @@ function AtImport(options) {
     resolve: resolveId,
     load: loadContent,
     plugins: [],
+    addModulesDirectories: [],
   }, options)
 
   options.root = path.resolve(options.root)

lib/resolve-id.js

@@ -29,7 +29,7 @@ module.exports = function(id, base, options) {
 
   var resolveOpts = {
     basedir: base,
-    moduleDirectory: moduleDirectories,
+    moduleDirectory: moduleDirectories.concat(options.addModulesDirectories),
     paths: paths,
     extensions: [ ".css" ],
     packageFilter: function processPackage(pkg) {

test/fixtures/resolve-custom-modules.css

@@ -0,0 +1,7 @@
+@import "shared-fake";
+@import "shared-auto";
+@import "shared-nest";
+@import "shared-by-hand/style.css";
+@import "shared-use-dep";
+@import "shared-use-dep-too";
+@import "shared-use-dep" screen;

test/fixtures/resolve-custom-modules.expected.css

@@ -0,0 +1,8 @@
+.shared-fake{}
+.shared-auto{}
+.shared-nested{}
+.shared-byHand{}
+.shared-dep{}
+@media screen{
+.shared-dep{}
+}

test/resolve.js

@@ -54,3 +54,12 @@ test("should be able to consume npm package or local modules", t => {
     from: "fixtures/imports/foo.css",
   })
 })
+
+test("should be able to consume modules from custom modules directories", t => {
+  return compareFixtures(t, "resolve-custom-modules", {
+    path: null,
+    addModulesDirectories: [ "shared_modules" ],
+  }, {
+    from: "fixtures/imports/foo.css",
+  })
+})

test/shared_modules/shared-auto/index.css

@@ -0,0 +1 @@
+.shared-auto{}

test/shared_modules/shared-by-hand/style.css

@@ -0,0 +1 @@
+.shared-byHand{}

test/shared_modules/shared-dep/index.css

@@ -0,0 +1 @@
+.shared-dep{}

test/shared_modules/shared-fake/main.css

@@ -0,0 +1 @@
+.shared-fake{}