Implement mark of emacs mode

[aycabta]

Implemented by d1d4ac6.

I should also implement kill-region and copy-region-as-kill (em-kill-region and em-copy-region in Editline). I think vi-set-mark by ` and vi-goto-mark by m in vi command mode should be also implemented.

ref. http://www.mit.edu/afs.new/sipb/project/sipbsrc/src/zsh2.2/help/zle