fix(gganimate.Rmd): Use 'anim_save()'

* In `gganimate.Rmd` we now save {gganimate} animation
  with `gganimate::anim_save()` instead of trying
  to save and view animation with {knitr} hook.
* This avoids an error due to an upstream bug.

closes #108

DESCRIPTION

@@ -1,7 +1,7 @@
 Package: ggpattern
 Type: Package
 Title: 'ggplot2' Pattern Geoms
-Version: 1.1.0-6
+Version: 1.1.0-7
 Authors@R: c(person("Mike", "FC", role = "aut"),
              person("Trevor L.", "Davis", role = c("aut", "cre"),
                     email = "trevor.l.davis@gmail.com",

vignettes/.gitignore

@@ -1,2 +1,3 @@
 *.html
 *.R
+images/*.gif

vignettes/gganimate.Rmd

@@ -15,6 +15,7 @@ knitr::opts_chunk$set(
   fig.width = 6,
   fig.height = 4
 )
+require("gganimate", quietly = TRUE)
 ```
 
 
@@ -58,13 +59,8 @@ p <- ggplot(df, aes(trt, outcome)) +
 p <- p + transition_states(time, transition_length = 2,
                     state_length = 0, wrap = FALSE)
 
-animate(p, nframes = 60, fps = 20)
-
+anim_save("images/gganimate.gif", p, nframes = 60, fps = 20)
 }
 ```
 
-Future
-------------------------------------------------------------------------------
-
-* Include more examples
-
+<img src="images/gganimate.gif" alt="ggpattern + gganimate">