Add AFINN

[bkmgit]

https://github.com/red-data-tools/red-datasets/issues/67

• Probably routine to download the dataset can be made cleaner.
• The dataset consists of two data files, only the newest one is used.

[kou]

Thanks. I've merged this with some changes:

• Use AFINN/AFINN-README.txt for description
• Improve naming
• Adjust styling