add Windows support

Windows _should_ be supported now, but I don't really have access to a
Win box, so..

Feedback is much appreciated!

autoload/startify.vim

@@ -29,7 +29,7 @@ function! startify#save_session(...) abort
     echo 'The session directory does not exist: '. g:startify_session_dir
     return
   endif
-  let spath = g:startify_session_dir .'/'. (exists('a:1')
+  let spath = g:startify_session_dir . startify#get_sep() . (exists('a:1')
         \ ? a:1
         \ : input('Save under this session name: ', '', 'custom,startify#get_session_names_as_string'))
         \ | redraw
@@ -50,7 +50,7 @@ function! startify#load_session(...) abort
     echo 'The session directory does not exist: '. g:startify_session_dir
     return
   endif
-  let spath = g:startify_session_dir .'/'. (exists('a:1')
+  let spath = g:startify_session_dir . startify#get_sep() . (exists('a:1')
         \ ? a:1
         \ : input('Load this session: ', '', 'custom,startify#get_session_names_as_string'))
         \ | redraw

plugin/startify.vim

@@ -9,7 +9,8 @@ endif
 let g:loaded_startify = 1
 
 " Init {{{1
-let g:startify_session_dir = resolve(expand(get(g:, 'startify_session_dir', '~/.vim/session')))
+let g:startify_session_dir = resolve(expand(get(g:, 'startify_session_dir',
+      \ has('win32') ? '$HOME\vimfiles\session' : '~/.vim/session')))
 
 augroup startify
   autocmd!