backspace characters in message() cause errors for pdf output

When I have an Rmd document with a chunk like:
```{r}
p_unload(dplyr)
```
The `p_unload` function inserts a backspace character "\b" into the message.  When the Rmd is converted to pdf, this error is shown:
```
Package inputenc Error: Unicode character (U+0008)
```
Suggested fix: Remove "\b" everywhere in `message()`s.
