Long Names

Hi,

If I have a long name I am trying to autocomplete, this no longer is useful. Would it be possible to allow for one of the following:

1. Allow the part already typed to not be included
2. Ellipsis the start not the end of the string
3. Allow for the text displayed in the UI to not be the text updates what is selected

While below is contrived, this happens with autocomplete of long chains of properties. Reproducer:

import random
import string

def randomString(stringLength=10):
    """Generate a random string of fixed length """
    letters = string.ascii_lowercase
    return ''.join(random.choice(letters) for i in range(stringLength))

def my_completer(c, text):
    longstr = 'AUTO_COMPLETE_A_VERY_LONG_STRING_BECAUSE_SOMETIMES_THIS_HAPPENS'
    if text.startswith(longstr):
        return [(longstr + randomString()).decode('utf8') for x in range(150)]
    return []

get_ipython().set_custom_completer(my_completer, 0)

Python 2.7.15
IPython 5.8.0
PromptToolkit: 1.0.15

Please also let me know if this is available in a later version of prompt_toolkit.

Start by running the supplied my_completer reproducer with the noted Python, IPython, and PromptToolkit versions, then trace the autocomplete UI from set_custom_completer. Done means long completion candidates remain usable and the displayed text can be shortened without changing the selected completion.