Make 'seconds' field optional about cron

[anton-dutov]

@zslayton

May be use proxy object 'ScheduleBuilder'

let s = Schedule::builder() // ScheduleBuilder::default()
    .with_seconds(true)
    .with_years(true)
    .parse(...)

[zslayton]

Importantly, expressions that do not include seconds but which do include years would be illegal.

To clarify this, the parser would treat any expression with 6 fields as having years. It would be impossible to detect the user's intent in some cases (e.g. * * * * * *), so this would simply be noted in the documentation.

May be use proxy object 'ScheduleBuilder'

I'm open to this approach. There are other settings that it would be nice to support -- e.g. whether day_of_week should be 0-indexed or 1-indexed, which varies between implementations. (This is relevant to #37.)