Cron string validation #126

This is another nice-to-have: a crontab(5) validator.

Some challenges: crontab(5) has a standard minimal language, but lots of cron implementations provide extensions like @hourly, as well as custom field values.

If third party dependencies aren't an issue, both crontab and croniter provide validation methods:

from crontab import CronSlices
CronSlices.is_valid("* * * * *")

import croniter
croniter.is_valid("* * * * *")

It looks like the former supports more extensions than the latter.