Allow setting the ON DELETE behaviour of foreign key constraints

[andrew-farries]

Add support for setting the ON DELETE behaviour of a foreign key constraint.

An example migration that uses the behaviour is:

{
  "name": "21_add_foreign_key_constraint",
  "operations": [
    {
      "alter_column": {
        "table": "posts",
        "column": "user_id",
        "references": {
          "name": "fk_users_id",
          "table": "users",
          "column": "id",
          "on_delete": "CASCADE"
        },
        "up": "(SELECT CASE WHEN EXISTS (SELECT 1 FROM users WHERE users.id = user_id) THEN user_id ELSE NULL END)",
        "down": "user_id"
      }
    }
  ]
}

The valid options for on_delete are CASCADE, SET NULL, RESTRICT, or NO ACTION. If the field is omitted, the default is NO ACTION,

Fixes #221