Add new option `ignorePackages`

[artch]

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:

bower: {
  dev: {
    dest: 'public/',
    options: {
      ignorePackages: ['jquery']
    }
  }
}