Support remaining time in `format_timedelta`

[homeworkprod]

Babel supports positive and negative timedeltas. From their code (babel/dates.py):

>>> format_timedelta(timedelta(hours=1), add_direction=True, locale='en')
'in 1 hour'
>>> format_timedelta(timedelta(hours=-1), add_direction=True, locale='en')
'1 hour ago'

Flask-Babel, however, only supports elapsed time. It subtracts the timedelta value from the current time:

if isinstance(datetime_or_timedelta, datetime):
    datetime_or_timedelta = datetime.utcnow() - datetime_or_timedelta

It would be great to properly handle dates in the future as well.

How about something like this?

if isinstance(datetime_or_timedelta, datetime):
    now = datetime.utcnow()
    if datetime_or_timedelta > now:
        datetime_or_timedelta = datetime_or_timedelta - now
    else:
        datetime_or_timedelta = now - datetime_or_timedelta