Add 'Applied' field to migration table to record timestamp when migration is run

[dmnyu]

I don't know if this would be wanted, as it seems to be common practice to put some kind of ad hoc timestamp in the ID field of a migration. I added an 'applied' field to the migrations interface here ,https://github.com/dmnyu/gormigrate/tree/applied-field, that records a RFC3339 timestamp of when the migration is applied to a database. Happy to open PR if this is wanted.