Allow for overriding only specific Mustache templates/partials. (#1719)

* Allow for overriding only specific templates/partials. Resolves  #1450.

bin/gollum

@@ -149,7 +149,7 @@ MSG
     'Can be set to \'gravatar\' or \'identicon\'. Default: standard avatar.') do |mode|
     wiki_options[:user_icons] = mode.to_s
   end
-  opts.on('--template-dir [PATH]', 'Specify custom mustache template directory.') do |path|
+  opts.on('--template-dir [PATH]', 'Specify custom mustache template directory. Only overrides templates that exist in this directory.') do |path|
     wiki_options[:template_dir] = path
   end
   opts.on('--template-page', 'Use _Template.{ext} as a template for new pages.') do
@@ -271,7 +271,6 @@ else
   Precious::App.set(:environment, ENV.fetch('RACK_ENV', :production).to_sym)
   Precious::App.set(:gollum_path, gollum_path)
   Precious::App.set(:wiki_options, wiki_options)
-  Precious::App.settings.mustache[:templates] = wiki_options[:template_dir] if wiki_options[:template_dir]
 
   if cfg = options[:config]
     # If the path begins with a '/' it will be considered an absolute path,