Add cursor shape handler

Here is a function which turns-on vi-mode dependent cursor shape.
The terminal determination is rather straightforward (need improvement).
The iTerm commands should coincide with konsole ones. So one need to add a string detecting iTerm and setting terminal to iterm in order to make it work in iTerm.

The commands are from:
http://vim.wikia.com/wiki/Change_cursor_shape_in_different_modes
http://vim.wikia.com/wiki/Configuring_the_cursor

I was not able to change cursor shape in gnome-terminal and xfce4-terminal as it written above.