`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.

Reproduce el problema desde .storybook/config.js, centrándote en la llamada a require.context y en la opción exclude mostrada. Inspecciona cómo se aplica la exclusión cuando los archivos se cargan mediante ese contexto. Se considera terminado cuando el archivo que se pretende excluir no se transforma, mientras que los demás archivos cargados mediante el contexto siguen funcionando.

Escrito por el modelo de indexación a partir del texto del issue.