Use glob pattern for file / folder associations

This icon set is awesome; thanks for all the hard work so far, it's fantastic.

Further to the great work you've already done, it would be cool if I could set all files within a folder to use a certain icon; e.g:
```
"material-icon-theme.files.associations": {
  "**/reducers/*.js": "redux-reducer"
},
```

Keep up the good work.