expose new ref-stats functionality via cli

now you can use the -t option to joinx ref-stats to specify what to
search for (long form: --token). The default behavior of searching for
a/t, c/g, and cg (CpG) sites remains, but can be specified explicity
like:

joinx ref-stats in.bed in.fa -t a/t -t c/g -t cg

note that more arbitrary options like -t a/cg -t acgt/tgca are legal.
use hash instead of tree for fasta sequence index