not auto-save session file when vim quit, add keymap to explicitly do
that. Since vim can be used to handy editor at ease, that means usually
it's just use "mvim junk.txt" to edit a file, save and quit, then
vim_session file will remember that junk.txt only, which makes no sense,
so better do save/load session file explicitly.

<S-F2> to save, <S-F3> to load.