Type definition for config.start_date and config.end_date

[DanielRose]

In the type definition file, the configuration values GanttConfigOptions.start_date and GanttConfigOptions.end_date are set as Date. However, they are undefined when the Gantt is initialized.

Since they are typed without undefined, this also means that the values cannot be set to undefined again in Typescript code (at least when strictNullChecks is true). Could this be fixed in a future version?