Require column matching while allowing empty sheets

[puchm]

Hello,

This is such a great and well made library. Great work!

One thing I have noticed is that it is apparently not possible to require that a column is matched with one in the imported sheet while still allowing empty cells inside of that column.

When you add this validation to a column definition and the user doesn't match it, the user gets prompted before proceeding to the next step:

{
  rule: 'required',
  level: 'info'
}

However, in the data validation step, this will lead to empty fields inside of said columns being highlighted.

I would propose adding a required attribute to the field definition to handle required columns separately from required values in cells.