Filter _Template content. Resolves #1603 and #1640. (#1612)

* Filter _Template content. Resolves #1603 and #1640.

* Introduces a Gollum::TemplateFilter class with methods to easily add _Template Filters.

* Adds support for relative _Template pages with fallback to _Template in root.

lib/gollum.rb

@@ -33,4 +33,20 @@ module Gollum
       super(message || "Cannot write #{@dir}/#{@attempted_path}, found #{@dir}/#{@existing_path}.")
     end
   end
+  
+  class TemplateFilter
+    @@filters = {}
+
+    def self.add_filter(pattern, &replacement)
+      @@filters[pattern] = replacement
+    end
+
+    def self.apply_filters(data)
+      @@filters.each do |pattern, replacement|
+        data.gsub!(pattern, replacement.call)
+      end
+      data
+    end
+  end
+
 end