Add back dplyr and ggplot2 as core deps?


An old blog post no longer works because of this. If it is of interest, editing these posts to add 

```r
library(tidyverse)
```

would be a good idea too!


Have you check rev-dep? if yes, does any package depend on the fact that tidyquant on Cran loads part of the tidyverse? (e.g. in examples or vignettes)

If so, I wouldn't mind opening PRs there to make needed adjustments!