allowed options.packageInfo to overwrite default packageInfo

Allowed ``options.packageInfo`` to overwrite default ``packageInfo`` so that environment specific options can be switched using ``grunt-apidoc``'s task specific options.

The following example grunt task configuration can be used for ``apidoc:dev``, ``apidoc:staging``, ``apidoc:production`` tasks.
```
dev: {
  src: "api/",
  dest: "doc/",
  options: {
    packageInfo : {
      sampleUrl: "https://localhost:6443"
    }
  }
},
staging: {
  src: "api/",
  dest: "doc/",
  options: {
    packageInfo : {
      sampleUrl: "https://staging.api.domain:6443"
    }
  }
},
production: {
  src: "api/",
  dest: "doc/",
  options: {
    packageInfo : {
      sampleUrl: "https://api.domain"
    }
  }
}
```