Match against options.pattern when recursing src dir

I couldn't get `blanket.options.pattern` to be respected and didn't see where it should be processed in the `grunt-blanket` source.

Is this the right approach to introduce it?

For context, here's my Gruntfile.js blanket section:

```javascript
    blanket: {
      options: {
        pattern: "!app/javascripts/app/react_components/**/*"
      },
      dev: {
        files: {
          "cov/app/javascripts/app": ["app/javascripts/app/"]
        }
      }
    },
```

(The instrumentation doesn't work on the JSX React components.)

Thanks!