Negative path support for glob parameter into .groc.json 

Did I miss something or glob parameter do not support negative paths as exclude from array into `.groc.json`?

Some example to clear minds:

```
"glob": [
  "src/**/*.coffee", "!src/**/*.spec.coffee",
  "gulp/**/*.coffee", "!gulp/**/*.spec.coffee",
  "README.md"
],
...
```
