Don't hide equations, except in MarkdownMML and MarkdownPNG

In other engines, e.g. textile, the equation delimiters have no
special meaning. So they should not be used to hide content from
wiki processing.

lib/wiki_content.rb

@@ -120,6 +120,7 @@ class WikiContent < String
 
   DEFAULT_OPTS = {
     :active_chunks       => ACTIVE_CHUNKS,
+    :hide_chunks         => HIDE_CHUNKS,
     :engine              => Engines::MarkdownMML,
     :engine_opts         => [],
     :mode                => :show
@@ -138,6 +139,8 @@ class WikiContent < String
     @options[:engine] = Engines::MAP[@web.markup]
     @options[:engine_opts] = [:filter_html, :filter_styles] if @web.safe_mode?
     @options[:active_chunks] = (ACTIVE_CHUNKS - [WikiChunk::Word] ) if @web.brackets_only?
+    @options[:hide_chunks] = (HIDE_CHUNKS - [Literal::Math] ) unless
+                  [Engines::MarkdownMML, Engines::MarkdownPNG].include?(@options[:engine])
 
     @not_rendered = @pre_rendered = nil
 
@@ -169,7 +172,7 @@ class WikiContent < String
     copy = WikiContentStub.new(self, @web, @options)
     @options[:engine].apply_to(copy)
 
-    copy.inside_chunks(HIDE_CHUNKS) do |id|
+    copy.inside_chunks(@options[:hide_chunks]) do |id|
       @chunks_by_id[id.to_i].revert
     end
   end