feat(SassPlugin): Shared resource files in the config

Author: nchanged

docs/plugins/sass.md

@@ -91,6 +91,54 @@ fuse.plugin(
 );
 ```
 
+## Resource file
+
+To avoid setting resource file (which could contain variable and other shared
+resources) on each file, it's possible to define it in the config.
+
+```js
+plugins: [
+  [
+    SassPlugin({
+      resources: [{ test: /.*/, file: "resources.scss" }],
+    }),
+    CSSPlugin(),
+  ],
+];
+```
+
+In the example above, all the files in the project, including node_modules will
+share the same resource file which in our case located next to `fuse.js`. It
+would also understand an absolute path.
+
+For example, having an SCSS file with the contents:
+
+`project/src/index.scss`
+
+```scss
+body {
+  background-color: $color1;
+}
+```
+
+and a resource file in `project/resources.scss`
+
+```scss
+$color1: orange;
+```
+
+Will result in the following output:
+
+```scss
+@import "../resources.scss";
+body {
+  background-color: $color1;
+}
+```
+
+As you can see here, the resource file is resolved automatically relatively to
+the file's original location.
+
 ## Macros
 
 Macros is a unique feature available only in `FuseBox` to give you more

src/plugins/stylesheet/SassPlugin.ts

@@ -2,13 +2,15 @@ import { File } from "../../core/File";
 import { WorkFlowContext, Plugin } from "../../core/WorkflowContext";
 import * as path from "path";
 import { Config } from "../../Config";
+import { ensureAbsolutePath } from "../../Utils";
 
 export interface SassPluginOptions {
 	includePaths?: string[];
 	macros?: { [key: string]: string };
 	importer?: boolean | ImporterFunc;
 	cache?: boolean;
 	header?: string;
+	resources?: [{ test: RegExp; file: string }];
 	indentedSyntax?: boolean;
 	functions?: { [key: string]: (...args: any[]) => any };
 }
@@ -63,6 +65,19 @@ export class SassPluginClass implements Plugin {
 			file.contents = this.options.header + "\n" + file.contents;
 		}
 
+		if (this.options.resources) {
+			let resourceFile;
+			this.options.resources.forEach(item => {
+				if (item.test.test(file.info.absPath)) {
+					resourceFile = item.file;
+				}
+			});
+			if (resourceFile) {
+				const relativePath = path.relative(file.info.absDir, ensureAbsolutePath(resourceFile));
+				file.contents = `@import "${relativePath}";` + "\n" + file.contents;
+			}
+		}
+
 		const options = Object.assign(
 			{
 				data: file.contents,