Config for ignoring undefined attributes

Hey there,

I've written a possible solution to task #95.

Now attributes can be ignored when they are undefined in 3 ways.

1. By setting the `ignoreEmpty` option to `true`:
    ```js
    Plates.Map({
      create: true,
      ignoreEmpty: true
    });
    ```

2. By specifying a particular attribute:
    ```js
    Plates.Map({
      create: true,
      ignoreEmpty: 'id'
    });
    ```

3. By selecting a few attributes with an array:
    ```js
    Plates.Map({
      create: true,
      ignoreEmpty: ['id', 'data-toggle']
    });
    ```