Custom type support feature

[Dejan Lokar]

Hi,

I'm using the rowserrcheck plugin in a project and I'm looking for a way to get the same check for a custom iterator.

Example:

type CustomIter struct {}
func (*CustomIter) Err() error {...}
...

Would you be interested in extending this linter to accept a list of types to check in addition to the Rows? I can prepare a PR.