Add a GUI option for creating the checkerboard in current layer

Hi Jeff,

Thanks for this great plugin. 

I needed a feature that creates the checkerboard in current selected layer. If you're inclined to integrate the feature, this pull request does that. 

Cheers
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(https://cloud.githubusercontent.com/assets/896157/4351791/ca062a1e-4214-11e4-9955-8eed9437b693.png)