Allow other time zone with `CRONUT_BASE_TIME_ZONE`

For groups whose offices are not in EST, this change allows for the
specification of an alternate time zone to use for scheduling through an
environment variable, `CRONUT_BASE_TIME_ZONE`.

Other changes:
* Using `strftime`'s `%Z` parameter to abbreviate time zones. This does result
  in the standard times being `EDT` instead of `EST`, but does allow for
  interface and alert output to use preferred time over a hard-wired zone.