Add --collapse-tree command line option

bin/gollum

@@ -72,6 +72,10 @@ opts = OptionParser.new do |opts|
   opts.on("--show-all", "Shows all files in file view. By default only valid pages are shown.") do
     wiki_options[:show_all] = true
   end
+
+  opts.on("--collapse-tree", "Collapse file view tree. By default, expanded tree is shown.") do
+    wiki_options[:collapse_tree] = true
+  end
 end
 
 # Read command line options into `options` hash