feat: add interface filtering support for mock generation with -interface flag

[tomMoulard]

This PR allows the filtering of generated interfaces when used in conjunction with the -source option.

This means that when generating mocks for a file, you can specify which interface you want to generate mocks on.

For example:

// file interfaces.go
type PiniaColada interface {
	Rhum() string
	Pine(apple string)
	Coconut()
}

type shirleyTemple interface {
	ale(ginger string)
	Grenadine()
	GetCherry() (maraschino string)
}

Running the following would only generate mocks for the PiniaColada interface.

mocktail -source interfaces.go -interface PiniaColada

Also note that the -interface can be provided with a comma (,) separated value to tell mocktail that you want to generate multiple mocks in a file.

Additional information

This PR fixes #9.

This PR is based on #33, and will need this PR to be approved/merged to do the same on this PR.

Thanks !