Update news

NEWS.md

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+# xaringanthemer 0.3.0
+
+* Added `background_header_auto` parameter that adds a background under the
+  first `h1` element (i.e. `# Slide Title`) on a slide. When enabled, the
+  background is added automatically to slides, as long as they aren't
+  `inverse`, `center`, `middle`, or `bottom`-styled slides. When disabled,
+  you can still enable slide title headers by adding the `header_background`
+  class to a slide. (thanks, @Btibert3, #10)
+  
+* The underlying template now uses the `whisker` package.