Make view/table generator optional

[AlekSi]

Sometimes we want to generate code only for Struct/Record, and not for View/Table. For example, see this reform-db model.

In a magic comment, table name - (as in //reform:-) should generate Struct/Record, but skip View/Table code.

[uzzz]

I wanted to help with this task, so I checked the code and it appears that it's not that easy: interface Struct has View() View method (https://github.com/go-reform/reform/blob/e66d9634783b49cfa2f9a4105691e6b1ef7c35bc/base.go#L62), so the generation of the view cannot just be skipped without API changes.

As soon as NextRow does not actually need .View() the possible solution could be is to split Struct interface into two: "pure struct" and "struct with a view", that embeds "pure struct" and adds .View() and then change the signature of NextRow() (naming is something that needs discussion).

What do you think?