Allow user to specify an offset for base pair location label

[diane]

In order to make it easer to map some random fasta file back to the genome, it'd be useful to provide a way to add some constant offset to the indexes reported in the user interface.

For example if you've got a 100 base pair long sequence, the values reported for its position might be 32113100, 32113199 (assuming the offset was 32113100).

(Actually we might want to think about if it should be 0 based or 1 based?)