Add routine to produce ribosome profiles on mrna transcripts.
Add --log option to scatter command line to plot points on log axis
handle negative-strand genes and genes with low read-depth

- if the strand is on the minus strand, subtract all positions from transcript length.
- Let plot draw the vertical axis since it handles low-abundance cases better.
- don't try to round max vertical limit to a round number in the hundreds place because it screws up really low-abundance transcripts.