Extend WindowStats: add time left before reset

[drygdryg]

https://github.com/alisaifee/limits/blob/0946fdcd3b9509041a823d3bb1b63ea1de9ab6ee/limits/util.py#L36-L44

Expected Behaviour

Currently, WindowStats class contains information about reset time and remaining amount. Could we add the time remaining until the reset? This would be useful in some scenarios with user interaction (for example, Telegram bots).

I think time left before reset can be calculated in this way:

import time

from limits import parse
from limits.storage import MemoryStorage
from limits.strategies import FixedWindowRateLimiter

storage = MemoryStorage()
my_limiter = FixedWindowRateLimiter(storage)
limit = parse('1 per 30 seconds')
my_limiter.hit(limit, 'test')
window_stats = my_limiter.get_window_stats(limit, 'test')
time_before_reset = window_stats.reset_time - time.time()
print(time_before_reset)