Add multiple files (one per table) code generation

TODO : write a proper (and relevant) junit test.
This is great. I was meaning to do that. Can you please split it in ddlCode and ddlDocs and move all the line breaks out of this to where they are called? Thx. Also could you please incorporate this https://github.com/slick/slick/issues/978 ? Because the current ddl code crashes the compiler for larger numbers of tables. The ticket describes the fix. That would be awesome.