Combine arrays when importing with mongoimport

It might be useful to have different options for merging arrays instead of just overwriting the whole array. Users could specify the field(s) for which they want a special array merge, and select a type of merge. Three possible merge types could be:

• addToSet
• append
• prepend

(See https://developer.mongodb.com/community/forums/t/combine-arrays-when-importing-with-mongoimport/11148)