This allows users to configure a set of types that may appear before
`context.Context`.

Notably, I think this rule is useful for allowing the `*testing.T` type
to come before `context.Context`, though there may be other uses (such
as putting a tracer before it, etc).

See mgechev#605 for a little more context on this.

Fixes mgechev#605

We can unindent if we make the above check more specific