`exclude` won't work with `require.context`

Hi,

I'll try to open a PR if I get no where, but I'd like to exclude a file from being transformed, but that file is required via require.context. when debugging, here is the output of stats when it tries to go over the file (config.js) that uses require.context:

PluginPass {
  _c: Map {},
  dynamicData: {},
  plugin:
   Plugin {
     _c: Map {},
     dynamicData: {},
     initialized: true,
     raw: {},
     key: 'react-css-modules',
     manipulateOptions: [Function: manipulateOptions],
     post: undefined,
     pre: undefined,
     visitor:
      { _exploded: {},
        _verified: {},
        ImportDeclaration: [Object],
        JSXElement: [Object],
        Program: [Object] } },
  key: 'react-css-modules',
  file:
   File {
     _c: Map {},
     dynamicData: {},
     pipeline: Pipeline {},
     log:
      Logger {
        filename: '/Users/mshertzberg/Gizmodo/kinja-components/.storybook/config.js',
        file: [Circular] },
     opts:
      { filename: '/Users/mshertzberg/Gizmodo/kinja-components/.storybook/config.js',
        filenameRelative: '/Users/mshertzberg/Gizmodo/kinja-components/.storybook/config.js',
        inputSourceMap: undefined,
        env: [Object],
        mode: undefined,
        retainLines: false,
        highlightCode: true,
        suppressDeprecationMessages: false,
        presets: [],
        plugins: [Object],
        ignore: [],
        only: undefined,
        code: true,
        metadata: true,
        ast: true,
        extends: undefined,
        comments: true,
        shouldPrintComment: undefined,
        wrapPluginVisitorMethod: undefined,
        compact: 'auto',
        minified: false,
        sourceMap: false,
        sourceMaps: false,
        sourceMapTarget: 'config.js',
        sourceFileName: '.storybook/config.js',
        sourceRoot: '/Users/mshertzberg/Gizmodo/kinja-components',
        babelrc: false,
        sourceType: 'module',
        auxiliaryCommentBefore: undefined,
        auxiliaryCommentAfter: undefined,
        resolveModuleSource: undefined,
        getModuleId: undefined,
        moduleRoot: '/Users/mshertzberg/Gizmodo/kinja-components',
        moduleIds: false,
        moduleId: undefined,
        passPerPreset: false,
        parserOpts: false,
        generatorOpts: false,
        basename: 'config' },
     parserOpts:
      { sourceType: 'module',
        sourceFileName: '/Users/mshertzberg/Gizmodo/kinja-components/.storybook/config.js',
        plugins: [Object] },
     pluginVisitors: [ [Object] ],
     pluginPasses: [ [Object] ],
     metadata: { usedHelpers: [], marked: [], modules: [Object] },
     dynamicImportTypes: {},
     dynamicImportIds: {},
     dynamicImports: [],
     declarations: {},
     usedHelpers: {},
     path:
      NodePath {
        parent: [Object],
        hub: [Object],
        contexts: [Object],
        data: {},
        shouldSkip: false,
        shouldStop: false,
        removed: false,
        state: undefined,
        opts: [Object],
        skipKeys: {},
        parentPath: null,
        context: [Object],
        container: [Object],
        listKey: undefined,
        inList: false,
        parentKey: 'program',
        key: 'program',
        node: [Object],
        scope: [Object],
        type: 'Program',
        typeAnnotation: null },
     ast:
      Node {
        type: 'File',
        start: 0,
        end: 722,
        loc: [Object],
        program: [Object],
        comments: [],
        tokens: [Object] },
     code: 'import \'preact/devtools\';\nimport {\n\tconfigure,\n\taddDecorator,\n\twithCentered,\n\twithKnobs,\n\tsetOptions\n} from \'../config/storybook\';\nimport { StyletronHOC } from \'../config/styletron\';\n\nfunction loadStories() {\n\tconst req = require.context(\'../components\', true, /story.js$/);\n\treq.keys().forEach(filename => req(filename));\n}\n\naddDecorator(getStory => <StyletronHOC>{getStory()}</StyletronHOC>);\naddDecorator(withCentered);\naddDecorator(withKnobs);\n\nsetOptions({\n\tname: \'kinja-components\',\n\turl: \'https://github.com/gawkermedia/kinja-components\',\n\tgoFullScreen: false,\n\tshowLeftPanel: true,\n\tshowDownPanel: true,\n\tshowSearchBox: false,\n\tdownPanelInRight: true,\n\tsortStoriesByKind: true\n});\n\nconfigure(loadStories, module);\n',
     shebang: '',
     hub: Hub { file: [Circular], options: undefined },
     scope:
      Scope {
        uid: 16,
        parent: null,
        hub: [Object],
        parentBlock: [Object],
        block: [Object],
        path: [Object],
        labels: [Object],
        references: [Object],
        bindings: [Object],
        globals: [Object],
        uids: {},
        data: {},
        crawling: false } },
  opts:
   { generateScopedName: '[hash:base64:10]',
     webpackHotModuleReloading: true,
     exclude: 'story',
     filetypes: { '.scss': 'postcss-scss' } } }

I would imagine this requires a PR to implement?

Thanks.

Tái hiện vấn đề từ .storybook/config.js, tập trung vào lệnh gọi require.context và tùy chọn exclude được hiển thị. Kiểm tra cách việc loại trừ được áp dụng khi các tệp được tải qua context đó. Hoàn tất khi tệp dự kiến bị loại trừ không được biến đổi, trong khi các tệp khác được tải qua context vẫn tiếp tục hoạt động.