Extended headers options for the excel format

[H3nSte1n]

Description:
If you use the Excel format, you need to define the header row. There you can set various options.

Task:
Add new configuration options for the header.

New Options:

• optional (The gaps from the row can be empty)
• allow_repititions (Columns with the same name may exist)

Example:

{
   id: {
     match: 'ID'
   },
   description: {
     match: 'Beschreibung',
     optional: true,
   }.
   link: {
     match: 'Link',
     optional: true,
     allow_repetition: true
   }
}

ID  (Beschreibung)  Link    Link     Link
1                   ...     ...      ..