Marked-up Sequence Field doesn't handle negative strand

[laceysanderson]

Currently there is a warning if you try to view the marked-up sequence for an alignment with strand=-1. Instead we should actually handle this appropriately.

Specifically,

fmin	fmax	strand	complement	switch fmin/fmax
small	large	+1	no	no
small	large	-1	yes	no
large	small	-1	yes	yes