Extra param to find_kmers to specify a dictionary to store k-mers

[slowikj]

As the total algorithm efficiency may depend on the perfomance of accessing k-mer data, it is important to choose an appropriate dictionary. The performance might depend on the size of data that is to be stored. Surprisingly, a linear list performs better that a hash map when the total number of elements is relatively small. Thus, it may be useful to make it possible for users to specify a dictionary type on their own because they know better the input data that is provided to the algorithm.