Addd PDF plot output option

[diekhans]

It would be really useful if driver scripts passed through a --pdf option to generate publication-quality plots;

[arangrhie]

Hi @diekhans ! Thanks for the suggestion. Will consider this for the next round of release :)

For publication, I used to re-run the plot_spectra-cn.R with adjusted x y limits (-m -n options) to pretty-fy it and add the -pdf option at that stage.