Add timezone support

[P4sca1]

At the moment, the cron jobs are checked against the local time.
When constructing a Schedule an optional timezone parameter could be used.
parseCronExpression would then get a second parameter timezone which is passed to the Schedule constructor.
For ease of use, it would be great if time zone names would be supported, but we would need to include and maintain a database of timezones. Also time zone offsets are not static but change over time and throughout the year. Maybe the timezone parameter should be a function utcOffset that returns the desired utc offset. One could then use other libraries such as moment-timezone to get the desired offset and cron-schedule bundle size would not increased (or use a peer dependency?). We also need to consider what happens when utcOffset returns diifferent offsets in subsequent calls (which might be realistic due to summer time / DST).