`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.