showfigures option in preprint.cls

I inserted an option "showfigures" that disables `preprint.cls` normal behaviour of ignoring the figures for the following reasons.

When writing and proof-reading a paper, I find it useful to have my figures in my document. Also, JoN (for example) allows all figures to be included in the draft in an initial submission:

> Initial Submission - Figures may be included in a single PDF file that contains the manuscript and all tables and figures.