Add hook support

Today the package already know some special comments, I'm proposing to add hook points as special comments where plain Go code could execute along the migration. This is very useful during transformations caused by the migration.

-- +migrate Up
ALTER TABLE users ADD COLUMN confirmation BOOLEAN;
-- +hook addConfirmation

-- +migrate Down
ALTER TABLE users DROP COLUMN confirmation BOOLEAN;

Using the standalone cli, don't know how this could work, but, using the library is kinda easy:

type hook func(db *sql.DB, tx *sql.Tx) error

migrations := &migrate.MemoryMigrationSource{
    Migrations: []*migrate.Migration{
        &migrate.Migration{
            Id:   "123",
            Up:   []string{"CREATE TABLE people (id int)"},
            Down: []string{"DROP TABLE people"},
            Hook: map[string]hook{"addConfirmation": addConfirmation},
        },
    },
}

func addConfirmation(db *sql.DB, tx *sql.Tx) error {
  // custom logic here
  return nil
}

Start by tracing how the library parses existing special comments and executes migrations, then compare that path with the standalone CLI path. Define how named hooks are registered and invoked for both directions, and how errors and transaction state are handled; the work is done when the proposed hook example works without leaving the CLI behavior undefined.