Accept key sequence

## problem
I assigned many key combinations as shortcut key for non-terminal applications.
So I have less key combinations which can be used in peco.

## solution
I changed peco to accept key sequence.
By this feature, you can assign many handlers for peco without keymap conflict (with existing key combinations) by using some key combination as prefix.

And you can use Emacs-like key sequence after this change. 
ex) `C-x C-c`