Create empty migration template using migrate

[madnesspie]

Hi! Thanks for new migrations format, .sql is really convenient.

I have a little enhancement proposal. Sometimes a programmer needs to write migration manually. For example to define partial index or create a date migration. In this case a programmer should go to the REPL and generate the string then copy it and create a migration file.

I guess a more convenient way is to use --empty console option for aerich migrate command how it works in django. If you agree I can contribute it.

[long2ice]

That's a good point. Of course PR is welcome. And Makefile is useful to do contribution.