Added option to select specific layer.

[csorvagep]

The layer selection is added, but the command line interface uses only the layer-id which is not really user friendly, altough that is only the CLI. Unfortunately I was not able to test the user interface in standalone mode, but it worked fine with inkscape.

[seebk]

Thanks for your pull request. I like the idea to be able to select a specific layer. However, I am quite busy at the moment with other projects and can't promise when I find the time to review and merge your PR.

[oliver-pola]

Just info is missing that this belongs to #10