Identifiable!, BelongsTo!, HasMany! are brocken when used with custom_derive

[weiznich]

Using one of those 3 macros with custom_derive will result in the following error:

error: expected ident, found ,
  --> <diesel macros>:21:25
   |
21 | Identifiable ! { $ args , fields = [ $ ( $ fields ) * ] , } } ; (
   |                         ^

Example Code:

custom_derive! {
     #[derive(Identifiable)]
     #[table_name(users)]
     struct User {
         id: i32,
         name: String,
     }
}

This is most likely caused by custom_derive not passing custom attributes down to the macro. The documentation mentions clearly:

"Secondly, the macro is passed the entire item, _sans attributes_. It is the derivation macro's job to parse the item correctly."

We should change the syntax here to something like:

custom_derive! {
     #[derive(Identifiable(table_name=users))]
     struct User {
         id: i32,
         name: String,
     }
}

[sgrif]

#438