**Breaking change:** `Table.upsert` now supports a `lenient` parameter
(default to `None`). If set to `True` (or to a list of column names),
allows a foreign key to not be resoled and simply put a null value in
database. If not set raises `UnresolvedFK` exception when a foreign
key can not be resolved (the corresponding value is not found in the
database)
**New feature:** `--where` argument in the command line now accept
multiple expressions. `--where` can also be repeated multiple times.
**New feature:** Proper support for `not_null` parameter. All column
names given in a list will be made required in the table definition in
the database.
**New feature:** Add `Schema.drop` to drop all tables in schema.