Ability to define unique columns when creating a table

When creating a new table, it would be good to have an option to set unique columns similar to how not_null is set.

from sqlite_utils import Database

columns = {"mRID": str, "name": str}
db = Database("example.db")
db["ExampleTable"].create(columns, pk="mRID", not_null=["mRID"], if_not_exists=True)
db["ExampleTable"].create_index(["mRID"], unique=True, if_not_exists=True)

So something like this would add the UNIQUE flag to the table definition.

db["ExampleTable"].create(columns, pk="mRID", not_null=["mRID"], unique=["mRID"], if_not_exists=True)

CREATE TABLE ExampleTable (
    mRID TEXT PRIMARY KEY
              NOT NULL
              UNIQUE,
    name TEXT
);

Start at the Database table create() entry point shown in the issue and trace how not_null and pk are turned into the CREATE TABLE statement. Add support for the unique option and verify that the generated schema includes UNIQUE for the requested columns while preserving the existing create_index behavior.