add kmer_continuous_count

this tool will count continuously, instead of line by line. The way that
this works out is something like this:

test.fa
> header 1
AAAAATTTTT
> header 2
GGGGGAAAAA

counting 6 mers, the program will count TTTGGG,  TTGGGG, TGGGGG, like
there was no header seperating them. This can be useful for certain
tyeps of processing, like when the sequences are continuous from a
genome.

initial commit