skip struct fields with incompatible types

[mastercactapus]

This PR changes the behavior of bindObject to ignore fields that cannot be automatically mapped from a struct. In the case of a type mismatch, the generated resolver will now require a method for the field, as if there were no matching field on the struct to begin with. This allows the logic for mapping field values to be provided as-needed, making cases like #82 hopefully easier to work with.

Previously this case would result in a fatal error (code gen would fail with a type mismatch error).

---

Example scenario (with this change):

Schema:

type User {
  createdAt String
  listOfEnums [FooEnum]
  name String
}
enum FooEnum {
  bin, bar, baz
}

Application struct (mapped with types.json):

type ListOfEnum []FooEnum
type User struct {
  Name string
  CreatedAt time.Time
  Foo ListOfEnum
}

Result:

type Resolvers interface {
  User_createdAt(ctx context.Context, obj *user.User) (*string, error)
  User_listOfEnums(ctx context.Context, obj *user.User) ([]FooEnum, error)
}

One could then simply define User_createdAt to map time.Time -> *string and User_listOfEnums to map ListOfEnum -> []FooEnum.

[vektah]

I think this makes sense, if gqlgen cant map directly between two types asking the user to implement a resolver seems like the correct thing to do.