zshrc - add key bindings for moving forward and backward 1 char

Signed-off-by: Pang Yan Han <pangyanhan@gmail.com>
zshrc - use Ctrl-d for toggling between insert and cmd modes

Since it is not a used shortcut and is relatively convenient.
Based on http://www.bewatermyfriend.org/media/vi-mode.zsh

Signed-off-by: Pang Yan Han <pangyanhan@gmail.com>