added Completion objects support for Word and Nested completers

[LucasParsy]

Allow to easily style WordCompleter and NestedCompleter completion prompts

obtained with this code example:

#!/usr/bin/env python3

from prompt_toolkit import prompt
from prompt_toolkit.completion import Completer, NestedCompleter, Completion

completer = NestedCompleter.from_nested_dict({
    Completion('colors', style="bold italic"): {
        Completion('blue', style="fg:ansiblue"): None,
        Completion('red', style="fg:ansired"): None,
        Completion('green', style="fg:ansigreen"): None
    },
    'normal': {'classic', 'strings', 'working'}
})

text = prompt('# ', completer=completer)
print('You said: %s' % text)

Also fixed bug where Completion could not be hashable. but there may be some risks of duplicates, please review.

My use case:
I just finished Advent of code 2019 (day 25), wich is a text-adventure game.
I wanted create a prompt which gives me completion of all available actions (in the style "go south", "take matches")
But I wanted the game to hint me for certain actions, like greying "south" if I already went there, or highlight some items in red to mark some sort of danger.