Bit of minor fixup for ggcorr - breaks when the range is narrow, greek letter rho when printing to latex

1.  Allows user to specify number of breaks shown in legend, and drops levels not in use.  Otherwise, the plotting function has little utility for correlation charts that don't cover a wide range, which in my experience is most of them.

2.  Added "latexify" option, which should title the legend with the greek character rho if it is used with the tikZ output device and rmarkdown.  Left this as undocumented feature.