add g:twcmd_focus_after_closing option

- g:twcmd_restore_prevfocus has been changed to g:twcmd_focus_after_closing
  which can be one of the string values, 'prev_win_curr_tab', 'prev_win_tab',or 'none'.
- change default focusing behavior to 'jump to the lastest window of the current tab'
  when a window is closed. ('prev_win_curr_tab')