Guard against uninstalled packages in rmarkdown template

[Garrick Aden-Buie]

• Set fig.show to TRUE when showtext is installed.

  xaringanthemer/inst/rmarkdown/templates/xaringanthemer/skeleton/skeleton.Rmd

  Line 28 in ce808c7

     fig.show = TRUE, 

• cli

  xaringanthemer/inst/rmarkdown/templates/xaringanthemer/skeleton/skeleton.Rmd

  Lines 147 to 149 in ce808c7

   ```{r message=TRUE} 
   cli::cli_alert_success("It worked!") 
   ``` 

• tibble

  xaringanthemer/inst/rmarkdown/templates/xaringanthemer/skeleton/skeleton.Rmd

  Lines 177 to 179 in ce808c7

   ```{r} 
   tibble::as_tibble(mtcars) 
   ``` 

• ggplot2

  xaringanthemer/inst/rmarkdown/templates/xaringanthemer/skeleton/skeleton.Rmd

  Lines 264 to 274 in ce808c7

   ```{r plot-example} 
   library(ggplot2) 
   (g <- ggplot(mpg) + aes(hwy, cty, color = class) + geom_point()) 
   ``` 
    
   --- 
    
   ```{r plot-example-themed} 
   g + xaringanthemer::theme_xaringan(text_font_size = 16, title_font_size = 18) + 
     ggtitle("A Plot About Cars") 
   ```