#105 bowtie2 implement for index concatenation

[Gholamali Rahnavard]

The idea would be to have multiple, independently indexed files (bug1.fnt -> bug1.bt2, bug2.fnt -> bug2.bt2) and do something like bowtie2 -x bug*.bt2.

This could be useful for larger genomes (e.g. human). One could have each chromosome indexed separately and then only map against the chromosomes of interest.