Allow --repo option & --migration-dir for phx.gen.schema

[MikaAK]

This allows us to provide a --repo flag to mix phx.gen.schema

It seems support is already built in for this with Mix.Phoenix.Schema so I simply added it to the opts in the format it seems is expected and made it utilise that repo for the migration path as well

Through options i've also added a --migration_dir which is passed in and used instead of the generated repo path

If this is wanted lemme know and I'll add tests to get it merged