New option: g:startify_session_sort

Sort your sessions by mtime by putting this in your vimrc:

    let g:startify_session_sort = 1

Closes #221.

autoload/startify.vim

@@ -570,6 +570,13 @@ function! s:show_sessions() abort
     call s:print_section_header()
   endif
 
+  if get(g:, 'startify_session_sort')
+    function! s:sort_by_mtime(foo, bar)
+      return getftime(a:foo) <= getftime(a:bar)
+    endfunction
+    call sort(sfiles, 's:sort_by_mtime')
+  endif
+
   for i in range(len(sfiles))
     let index = s:get_index_as_string(s:entry_number)
     let fname = fnamemodify(sfiles[i], ':t')

doc/startify.txt

@@ -116,6 +116,7 @@ default values.
     |g:startify_session_remove_lines|
     |g:startify_session_savecmds|
     |g:startify_session_savevars|
+    |g:startify_session_sort|
 
 ------------------------------------------------------------------------------
                                                         *g:startify_session_dir*
@@ -412,6 +413,14 @@ Example:
            \ 'silent !pdfreader ~/latexproject/main.pdf &'
            \ ]
 <
+------------------------------------------------------------------------------
+                                                       *g:startify_session_sort*
+>
+    let g:startify_session_sort = 0
+<
+Sort sessions by modification time (when the session files were written)
+rather than alphabetically.
+
 ------------------------------------------------------------------------------
                                                      *g:startify_custom_indices*
 >