Add +ESC key binding

Mainly for emacs users, I'd like to introduce a new form to key bindings with a Meta key with "^[".

Exapmle:

    bind generic ^[v move-page-up

In actuality, when press a key with a Meta(or Option key on Mac) key, terminal apps send a key sequence whose first key code is KEY_ESC. This commit add a trap to receive KEY_ESC and then get the main key code again.