Use PyStemmer (#217)

PyStemmer exposes bindings to libstemmer_c,
the core Snowball library written in C.
This can improve performance of word stemming.

build_docs.py

@@ -115,6 +115,7 @@ def requirements(self):
 
         return [
             "jieba",  # To improve zh search.
+            "PyStemmer~=2.2.0",  # To improve performance for word stemming.
             "-rrequirements.txt",
         ]