Allow customization of the Haskell name

[parsonsmatt]

This popped up at work. We have a database table that is being replicated across two databases. Converting that table to mpsGeneric would be a huge PITA, and we don't want to do it, so instead we're going to read the same .persistentmodels file twice - once for the main DB and once for the replicated DB.

While this will do the right thing, the generated type will be identical namewise, and this may be confusing. We'd like to customize the name of the Haskell datatype, similar to how we customize the name of the SQL table.