INIT: Subcommand to Scaffold Migrations for Modules

[julienrbrt]

ref: cosmos/b9-checkers-academy-draft#65 (comment)

Proposal

Add a subcommand to scaffold migrations boilerplate for a module.
The should be smart enough to bump the consensus version, register the migration in module.go and add boilerplate migrations code in a migration package, named after the consensus version (vX)