Update/Skipping row during data import into database

Current, it was only possible to import data into an empty database using SQLAlchemy. However, it is more often that the database is not empty and hence needs a bit extra logic to cope with existing data: 1) update existing items 2) skipping them. Let's give the choice to the developer.

Relevant issue: https://github.com/chfw/Flask-Excel/issues/7
