Your comments

The output format is pretty simple and the programmer can further edit it if they want to after the text is generated.

The hard part is that a migration should only include what has changed from the previous generate, and the programmer should be able to choose between generating a single migration for the last set of changes or generating one text file per table.  

It might be better or easier to generate the migration code for all changes since the last time migration code was generated, but let the programmer run "rails g migration mymigrationname" on their own machine to create an empty migration and then cut and paste the Vertabelo-generated code into the local migration fie.