Use :SS! to force overwriting a potentially existing session

This can be used to avoid the following prompt when using :SSave

  "Session already exists. Overwrite?  [y/n]"

References #295

autoload/startify.vim

@@ -200,7 +200,7 @@ function! startify#session_load(...) abort
 endfunction
 
 " Function: #session_save {{{1
-function! startify#session_save(...) abort
+function! startify#session_save(bang, ...) abort
   if !isdirectory(s:session_dir)
     if exists('*mkdir')
       echo 'The session directory does not exist: '. s:session_dir .'. Create it?  [y/n]'
@@ -240,7 +240,7 @@ function! startify#session_save(...) abort
   endif
 
   echo 'Session already exists. Overwrite?  [y/n]' | redraw
-  if nr2char(getchar()) == 'y'
+  if a:bang || nr2char(getchar()) == 'y'
     call startify#session_write(fnameescape(spath))
     echo 'Session saved under: '. spath
   else

plugin/startify.vim

@@ -56,8 +56,8 @@ function! s:extinction()
   endif
 endfunction
 
-command! -nargs=? -bar -complete=customlist,startify#session_list SSave   call startify#session_save(<f-args>)
-command! -nargs=? -bar -complete=customlist,startify#session_list SLoad   call startify#session_load(<f-args>)
+command! -nargs=? -bar       -complete=customlist,startify#session_list SLoad   call startify#session_load(<f-args>)
+command! -nargs=? -bar -bang -complete=customlist,startify#session_list SSave   call startify#session_save(<bang>0, <f-args>)
 command! -nargs=? -bar -bang -complete=customlist,startify#session_list SDelete call startify#session_delete(<bang>0, <f-args>)
 command! -nargs=0 -bar SClose call startify#session_close()
 command! -nargs=0 -bar Startify noautocmd enew | call startify#insane_in_the_membrane()