Can scan pattern of files

Currently, it's impossible to ask `grunt-jsduck` to scan a list of files given by a pattern
For example the following config in `Gruntfile.js` doesn't work:
```
        jsduck: {
            main: {
                src: ['./Resources/public/js/**/*.js'],
                dest: 'build/api/jsduck'
            }
        }
```

With this PR, this config now works. The fix has been inspired by the code from `grunt-jsdoc`