Probably something like this:

template< class Key, class Hash = std::hash<Key> > class BloomFilter;

I don't use c++ so I'm not sure how std:hash works or gets implemented, but the way that guava (Google's java library) does it is by passing in a key and a funnel object. The funnel object is essentially responsible for decomposing the object into a byte stream. The advantage of doing it this way rather than making the caller specify his own hash is that you can use murmurhash3 which you thought had the best properties for the bloom filter.

I updated the blog post with your suggestion; CDN should be updated soon :)