generate a tag for migration files to allow ... to work

[Jeff Hodges]

It would be cool if things like go test ./... could work in a codebase that uses goose. Right now, that will error out because the migration files generated by goose cause build errors (because of their missing main functions).

It would be cool if, by default, goose added "// +build" tags to its generated scripts to make it a little easier for folks to skip over them in their tooling.

The obvious way to do this is backwards incompatible, unfortunately. But perhaps someone has a better idea or the incompatibility is okay for other reasons.