Update Excel export to specify columns.

[wesleyzloza]

The user should be able to configure the layout / columns displayed in the Microsoft Excel document created using BOM.export(). A example configuration can be:

{
   columns: [
      {name: "Part Number", key: "pn", width: 1},
      {name: "Description", key: "desc", width: 3}
   ]
}

In this case width would be the column width in inches. The key property would bind to a key that should be available in every BOM item.