Render R code in skeleton when pkg is installed (#55)

NEWS.md

   a corresponding CSS rule setting the `font-weight` property of `<strong>`
   elements (thanks @matiasandina, #50).
   
+* The xaringanthemer slide skeleton (R Markdown template) will now render when
+  packages that are used in the template but not in xaringanthemer's Imports are
+  not installed (#52).
+
 * Set slide number font size in `rem` so that increasing the font size at the
   slide level doesn't result in giant slide numbers (#53).
 

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

   echo = TRUE,
   message = FALSE, 
   warning = FALSE,
-  fig.show = TRUE,
+  fig.showtext = requireNamespace("showtext", quietly = TRUE),
   hiline = TRUE
 )
 ```
 
 ---
 
-```{r message=TRUE}
+```{r message=TRUE, eval=requireNamespace("cli", quietly = TRUE)}
 cli::cli_alert_success("It worked!")
 ```
 
 ## Tables
 
 ---
+exclude: `r if (requireNamespace("tibble", quietly=TRUE)) "false" else "true"`
 
-```{r}
+```{r eval=requireNamespace("tibble", quietly=TRUE)}
 tibble::as_tibble(mtcars)
 ```
 
 ```
 
 ---
+exclude: `r if (requireNamespace("DT", quietly=TRUE)) "false" else "true"`
 
-```{r}
+```{r eval=requireNamespace("DT", quietly=TRUE)}
 DT::datatable(head(mtcars), fillContainer = FALSE, options = list(pageLength = 4))
 ```
 
 
 ---
 
-```{r plot-example}
+```{r plot-example, eval=requireNamespace("ggplot2", quietly=TRUE)}
 library(ggplot2)
 (g <- ggplot(mpg) + aes(hwy, cty, color = class) + geom_point())
 ```
 
 ---
 
-```{r plot-example-themed}
+```{r plot-example-themed, eval=requireNamespace("ggplot2", quietly=TRUE)}
 g + xaringanthemer::theme_xaringan(text_font_size = 16, title_font_size = 18) +
   ggtitle("A Plot About Cars")
 ```