`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 the issue from .storybook/config.js, focusing on the require.context call and the shown exclude option. Inspect how exclusion is applied when files are loaded through that context. Done means the intended excluded file is not transformed while the other context-loaded files continue to work.

Written by the indexing model from the issue text.