Add example for converting from readline-based key bindings to Prompt Toolkit

The keybinding example shows how to do some fancy things with key bindings (CtrlT to print "hello world"), so it's clear how to execute an arbitrary callable for a given key combination (or at least, for key combinations defined in from prompt_toolkit.keys.Keys), but if I just want to customize key combinations for existing cursor functions, where would I go? For instance, my .inputrc contains:

C-f: backward-word
C-g: backward-kill-word

The prompt-toolkit alternative should be something like:

from prompt_toolkit import prompt
from prompt_toolkit.key_binding.manager import KeyBindingManager
from prompt_toolkit.keys import Keys
manager = KeyBindingManager.for_prompt()

@manager.registry.add_binding(Keys.ControlF)
def _(event):
    # < Function call to move cursor backward one word>

@manager.registry.add_binding(Keys.ControlG)
def _(event):
    # < Function call to kill backward one word>

I'd be happy to add something like this to the keybinding example, if someone can tell me what functions to call. Cheers!

Start with examples/custom-key-binding.py and the KeyBindingManager and Keys entry points named in the issue. Find the existing cursor and kill-word functions that correspond to the two inputrc bindings, then update the example to show both conversions and verify that the documented key combinations perform the requested actions.