Add Xianxia genre

Immortal General:

In C dramas, both wuxia and xianxia dramas are shown in results when searching under the wuxia genre. It would make more sense to classify them.

This article made me understand the difference better.

If things go well, we should be able to fix and reorganize the genres and have those two as separate genres sometime this year. Until then, the Xianxia tag will stay under the Wuxia genre.