add single-chars to memo automatically

[ulf1]

https://github.com/ulf1/kshingle/blob/055a75c55bcacfbea7b4fd7c8c01d758866fe5d7/kshingle/cews.py#L263

if len(memo) == 0:
    memo = {k:v for k,v in db.items() if len(k)==1}