Provide a method to rollback all migrations?

[Reasno]

A method to rollback all migrations would be very helpful in testing. While working on library/shared code, it is impossible to know the first ID without doing an extra query.

[genofire]

I will create an PR, after we solve the problem like in #69

[guykra]

Hope that https://github.com/go-gormigrate/gormigrate/pull/70 will be approved soon 😄