gmig all down

[Ernest Micklei]

currently, executing gmig down <folder> will run the undo section of the last migration only.

This feature will allow you to run all undo sections of all migrations, one after the other.

[Vladimir Oganesov]

@emicklei hey Ernest. I just created a PR to add down-all command :)

[Ernest Micklei]

merged, thx