In response to issue #128 and for my needs, I added basic capability to set pch= and col=
for the points plotted for groups. There are still quite a few other graphical parameters hardcoded,
but I resisted the temptation to go further in this round.
added pch= control to plot.betadisper.R
added col= control to plot.betadisper() re: #128
Thanks @friendly. I'm just running checks locally before merging this.