New feature to reverse orientation of plots

[stephenrong]

I'm trying to do some comparative functional genomics across multiple species for a give gene. I'm wondering if it is possible to implement plotting of the reverse orientation, as in the "reverse" button on the UCSC Genome Browser. In the attached example for GADD45A, the second row is from a species where the orientation of this locus is reverse from the other four, making it harder to vertical compare the locus across species. My current plan is to trick plotgardener with an object with "reversed" coordinates, but this appears easier for the functions plotSignal and plotRanges than plotGenes, which requires a new txdb and thus assembly object.