Expose cron expression fields

[brunoluiz]

With the actual implementation the developer can't do other operations using the parsed cron expression. In this pull request I expose the Expression struct fields, which enables the developer to use the previously private fields.

Example:

expr := "0,10,20,30,40,50 4 * * *"
cron := cronexpr.MustParse(expr)
log.Info(cron.MinuteList)