Allow migration files to have `.up.sql` suffix and use --include-down false

[jvatic]

As a work-around, if you need your migrations to have the .up.sql suffix but don't want to have down migrations, run

sql-schema migration && git ls-files --others --exclude-standard | grep \\.down\\.sql$ | xargs -r rm

(i.e. immediately delete the generated .down.sql files.)