Add glob.exclude

Re. https://github.com/plumberjs/plumber-glob/issues/1

You would use this in a pipeline as follows:

``` js
[
  glob('test/files/file-*.js'),
  glob.exclude('test/files/file-1.js')
]
```

The [alternative APIs suggested](https://github.com/plumberjs/plumber-glob/issues/1) were tricky to implement. At the moment, the `glob` function returns a Plumber pipeline function. The following:

``` js
glob('test/files/file-*.js').exclude('test/files/file-1.js')
```

Suggests that the `glob` function would need to instead return an object which stores some state about the glob’s resources. I’m not sure if we want to do this?