app: flatten model config

Support flat structure of model config objects, where model options
are set as top-level properties.

Before:

    Customer: {
      dataSource: 'db',
      options: {
        base: 'User'
      }
    }

Now:

    Customer: {
      dataSource: 'db',
      base: 'User'
    }

/to @raymondfeng or @ritch please review