Add support for circular chromosomes

[evancofer]

At present, Selene assumes chromosomal linearity. Many prokaryotes have circular chromosomes. In the case of circular chromosomes, Selene should allow a sequence feature to wrap from the chromosome "end" in the FASTA file to the chromosome "start" in the FASTA file. It is also uncertain how users would specify bins that span these areas.

The easiest fix may be to pad the start and end by some fixed amount depending on the feature size.