New option: g:startify_session_persistence

Whenever you load/save a session, v:this_session is set automatically.
If this option is enabled and v:this_session exists, the session file
will be updated before exiting Vim.

References #32.

doc/startify.txt

@@ -142,6 +142,7 @@ The number of files to list.
 When the file Session.vim is found in the current directory, it will be shown at
 the top of all lists as entry [0].
 
+
 ============-
 
     let g:startify_session_autoload = 0
@@ -152,6 +153,14 @@ directory.
 
 Great way to create a portable project folder.
 
+
+============-
+
+    let g:startify_session_persistence = 0
+
+Automatically update sessions before exiting Vim.
+
+
 ============-
 
     let g:startify_change_to_dir = 1