Adding one-sided transitions

Now the transition isn’t any more „per slide“ but „per movement of a
slide“. So the slide can disappear in another way than it appeared.

README.md

@@ -474,6 +474,27 @@ The global presentation transition is set using the ```transition``` config valu
 </section>
 ```
 
+You can as well override only the appearing or the disappearing of the slide:
+
+```html
+<section data-transition="slide">
+    The train goes on … 
+</section>
+<section data-transition="slide"> 
+    and on … 
+</section>
+<section data-transition="slide-in fade-out"> 
+    and stops.
+</section>
+<section data-transition="fade-in slide-out"> 
+    (Passengers entering and leaving)
+</section>
+<section data-transition="slide">
+    And it starts again.
+</section>
+```
+
+
 Note that this does not work with the page and cube transitions.
 
 

css/reveal.scss

@@ -458,12 +458,14 @@ body {
 }
 @mixin transition-horizontal-past($style) {
   .reveal .slides>section[data-transition=#{$style}].past,
+  .reveal .slides>section[data-transition~=#{$style}-out].past,
   .reveal.#{$style} .slides>section:not([data-transition]).past {
     @content;
   }
 }
 @mixin transition-horizontal-future($style) {
   .reveal .slides>section[data-transition=#{$style}].future,
+  .reveal .slides>section[data-transition~=#{$style}-in].future,
   .reveal.#{$style} .slides>section:not([data-transition]).future {
     @content;
   }
@@ -471,12 +473,14 @@ body {
 
 @mixin transition-vertical-past($style) {
   .reveal .slides>section>section[data-transition=#{$style}].past,
+  .reveal .slides>section>section[data-transition~=#{$style}-out].past,
   .reveal.#{$style} .slides>section>section:not([data-transition]).past {
     @content;
   }
 }
 @mixin transition-vertical-future($style) {
   .reveal .slides>section>section[data-transition=#{$style}].future,
+  .reveal .slides>section>section[data-transition~=#{$style}-in].future,
   .reveal.#{$style} .slides>section>section:not([data-transition]).future {
     @content;
   }