adding snow_simplified_japanese_corpus
merging since the CI is fixed on master Thank you for the updates and merging!
Adding simplified Japanese corpus "SNOW T15" and "SNOW T23".
They contain original Japanese, simplified Japanese, and original English (the original text is gotten from en-ja translation corpus). Hence, it can be used not only for Japanese simplification but also for en-ja translation.

- http://www.jnlp.org/SNOW/T15
- http://www.jnlp.org/SNOW/T23