Reveal should be an outputFormat

Now any Hugo section can be configured to generate a presentation without altering its type, and the theme is more easily copied into existing sites without conflicting with any preexisting layouts.

exampleSite/content/_index.md

@@ -1,5 +1,6 @@
 +++
 title = "reveal-hugo example presentation"
+outputs = ["Reveal"]
 +++
 
 # reveal-hugo

exampleSite/content/example/_index.md

@@ -1,5 +1,6 @@
 +++
 title = "Example of a section presentation"
+outputs = ["Reveal"]
 reveal_theme = "moon"
 +++
 

exampleSite/content/home/body.md

@@ -22,7 +22,11 @@ weight = 20
 
 # reveal-hugo
 
-Create multiple presentations, one for each section in `content`.
+Create a presentation for any section by adding this to its `_index.md`:
+
+```toml
+outputs = ["Reveal"]
+```
 
 ---