`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 => {getStory()});\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.

.storybook/config.js から問題を再現し、require.context の呼び出しと、示されている exclude オプションに注目してください。そのコンテキストを通じてファイルが読み込まれる際に、除外がどのように適用されるかを調べてください。意図した除外対象のファイルが変換されず、他のコンテキスト経由で読み込まれたファイルは引き続き動作すれば完了です。

索引モデルが issue の本文から書いたものです。