Add support for ON UPDATE CURRENT_TIMESTAMP columns

[JanJakes]

The ON UPDATE CURRENT_TIMESTAMP is a special MySQL-specific construct that can be used in column definitions. There is no such functionality in SQLite. This pull request implements the ON UPDATE CURRENT_TIMESTAMP functionality for both CREATE and ALTER table commands using triggers.

With queries such as:

CREATE TABLE _tmp_table (
	id int(11) NOT NULL,
	created_at timestamp NULL ON UPDATE CURRENT_TIMESTAMP
);

ALTER TABLE _tmp_table ADD COLUMN updated_at timestamp NULL ON UPDATE CURRENT_TIMESTAMP;

The following triggers will be added:

-- created_at
CREATE TRIGGER "___tmp_table_created_at_on_update__"
AFTER UPDATE ON "_tmp_table"
FOR EACH ROW
BEGIN
  UPDATE "_tmp_table" SET "created_at" = CURRENT_TIMESTAMP WHERE id = NEW.id;
END;

-- updated_at
CREATE TRIGGER "___tmp_table_updated_at_on_update__"
AFTER UPDATE ON "_tmp_table"
FOR EACH ROW
BEGIN
  UPDATE "_tmp_table" SET "updated_at" = CURRENT_TIMESTAMP WHERE id = NEW.id;
END;

When the ON UPDATE is dropped using a query such as the following:

ALTER TABLE _tmp_table
CHANGE created_at created_at timestamp NULL,
CHANGE COLUMN updated_at updated_at timestamp NULL

The triggers are dropped as well. To ensure this always works, the trigger is always dropped when a column is being altered. Then, if ON UPDATE is present, it will be recreated; otherwise, it will remain dropped.

Resolves #148.

[bgrgicak]

Thanks for the PR @JanJakes!