Testing method support

[glerchundi]

What do you think about adding a method that executes just after the migration step and before increasing the current migration version? Mainly to be sure that everything worked as expected.

Something like:

&migrator.Migration{
	Name: "my migration",
	Func: func(tx *sql.Tx) error {
		// add migration logic here.
	},
	TestFunc: func(tx *sql.Tx) error {
		// add testing logic here.
        },
}

[lopezator]

I think this could be a good idea.

It would be nice if you could test this functionality and provide feedback or examples of usage.

I'll be glad to add this if it proves to be useful for the regular user of the tool.