Support FOREIGN KEY constraints

[frumpled]

Add support for FOREIGN KEY.

Ex/

CREATE TABLE Orders (
    OrderID int NOT NULL,
    OrderNumber int NOT NULL,
    PersonID int,
    PRIMARY KEY (OrderID),
    FOREIGN KEY (PersonID) REFERENCES Persons(PersonID)
);

As documented here:
https://www.w3schools.com/sql/sql_foreignkey.asp