Add Content-Appearance header

Allows switching between the "full-width" and "fixed" page layouts.

The "fixed" layout renders the page in a narrow column. (Confluence
default)

If not configured, it defaults to "full-width".

README.md

@@ -55,6 +55,13 @@ Also, optional following headers are supported:
 * (default) page: normal Confluence page - defaults to this if omitted
 * blogpost: [Blog post](https://confluence.atlassian.com/doc/blog-posts-834222533.html) in `Space`.  Cannot have `Parent`(s)
 
+```markdown
+<!-- Content-Appearance: (full-width|fixed) -->
+```
+
+* (default) full-width: content will fill the full page width
+* fixed: content will be rendered in a fixed narrow view
+
 ```markdown
 <!-- Sidebar: <h2>Test</h2> -->
 ```