Make Tilt renderer configurable

Nesta uses the Tilt gem to render Markdown, Haml, and Textile into HTML.
It uses the default options for each rendering library.

This patch allows a site's developers to configure whichever rendering
library they happen to be using, by passing those options through to a
particular renderer when it's instantiated.

To use this feature, override the `#render_config` method in your app.rb
file.

Closes #146.

CHANGELOG.md

@@ -4,6 +4,10 @@
 
  * Upgrade to Sinatra 3.1. (なつき)
 
+ * Tilt renderers (e.g. for Markdown, Haml, or Textile) are now configurable.
+   See #146 for details.
+   (Graham Ashton)
+
 ## 0.15.0 (11 July 2023)
 
  * Upgrade to Sinatra 3. (Graham Ashton)

lib/nesta/models/file_model.rb

@@ -184,8 +184,12 @@ def add_p_tags_to_haml(text)
 
     def convert_to_html(format, scope, text)
       text = add_p_tags_to_haml(text) if @format == :haml
-      template = Tilt[format].new { text }
+      template = Tilt[format].new(renderer_config(@format)) { text }
       template.render(scope)
     end
+
+    def renderer_config(format)
+      {}
+    end
   end
 end