Import existing data from CSV, Excel etc

When a user creates a table for the first time he might want do this from an existing data source like a CSV or Excel file. It must be possible to import these data source. Baserow should automatically suggest field types per field, but the user should still be able to manually define them.

Developers should be able to add custom data sources via plugins. If someone wants to build a connection with SQLite for example he should be able to do so.