Flag for interface exclusion

[tulzke]

Please add the ability to exclude interfaces, if source mode of mockgen is used.

This can be useful in the following case:

package segmentation

//go:generate mockgen -destination=segmenter_mock.go -package=segmentation -source=segmenter.go

// need to exlude this interface, because it is constraint
type Segment interface {
	~int
}

type Segmenter[V Segment] interface {
	Segment(code string) V
}

You can do this by passing a flag or using a comment like // gomock:exclude