Extension for the SD Method

[kurokida]

Dear developers,

I have extended the survey-likert plugin to enable the use of the SD method. For details, please refer to this commit. kurokida@fa6e085

I believe it is better to treat this as a feature enhancement of survey-likert rather than creating a separate plugin.
If you agree, I will submit a pull request.

I have also attached an image showing how it looks when used with the SD method.

Thank you for your consideration.

Best regards,
Daiichiro