Multi-threaded stemming?

First of all, thank you for the library!

I was wondering is it possible to use it with multi-threading? 

I've tried initializing several stemmers (`Stemmer()`), one for each thread, but it led to a segfault. I
'm not sure why - I thought it should have initialized separate stemmers.

Is there another workaround that can use to parallelize the stemming workload?