Add new option `ignorePackages`

A useful option for ignoring packages when we don't want them to be copied while they are still required by some other packages. For example, `Bootstrap` comes with `jquery` dependency, however it is widely popular to include `jquery` via CDN, so we don't want it to be a part of our build process.

Config sample:
```javascript
bower: {
  dev: {
    dest: 'public/',
    options: {
      ignorePackages: ['jquery']
    }
  }
}
```