Change log for prepplot

Version 1.0-1, April 2021
- added rmarkdown to Suggests, because of change in knitr

Version 1.0, April 2021
- changed defaults for two colours in prepplot: bg from "grey92" to par("bg") and col.stripes from "grey98" to "grey90" (like in stripes)
- modified vignette to adjust for the above changes and improve a few wordings

Version 0.7, April 2018
First version on CRAN