Use arrows for points in showDigitizedImage()

[droglenc]

All for the use of arrows rather than pch symbols for points in showDigitizedImage().

Should be able to do this with text() using unicode characters inside of intToUtf8(), pos= as with the annuli labels, and offset=0. The unicode characters to use are below

intToUtf8(c(9650,9660,9658,9668))

Will probably need to create some sort of catch with pch.show= to catch if the user says "arrows." Or, set a useArrow= logical argument.

[droglenc]

Also see https://stackoverflow.com/questions/21607151/obtaining-left-and-right-arrows-as-plot-symbols-in-r

[droglenc]

Will be in v0.2.1.