Add --number flag to dbmate down

[mg98]

It bugged me that I had to rollback migrations one by one if I needed to rollback multiple.
Also, the option to rollback all migrations at once (instead of drop) could be useful for CI testing.

[gregwebs]

This would interact poorly with #434 (probably throw an error if both are specified). It seems like this could be accomplished externally with a loop (not sure how annoying that is in your case).

[gregwebs]

#256 also tries to tackle this by specifying a target to migrate down to. I think the approach there could be safer and avoid accidental off by 1 errors, etc?

[mg98]

I'm currently not using this lib in any project so I'm not opinionated. I feel like such an option would be useful and more convenient than typing the version, especially when working collaboratively or on different branches. I don't see this being at particularly high risk for off-by-1 errors.

With regards to #434, you mean when both arguments were to be specified at once? Not sure, maybe.
Might take a look on it again depending how this discussion concludes ;)

[gregwebs]

I opened #438 which allows a syntax for specifying a range of migration. If you wanted to specify migrating down the last 2 versions, you could use -m -2.... But that needs to be combined with a flag or command that would allow for rolling back multiple migrations at once.