accept classifier interface in analyzer

[breml]

Currently the public functions in package github.com/google/capslock/analyzer accept a *interesting.Classifier as an argument. For external usage of this package, it would be beneficial, if these functions would instead accept an interface as follows:

type Classifier interface {
	FunctionCategory(pkg string, name string) cpb.Capability
	IncludeCall(caller string, callee string) bool
}

This would allow users of this package to write their own classifiers or to extend the provided classifiers e.g. by wrapping them (middleware approach).

I am happy to provide a PR for this.

[jcd2]

Good idea, thanks!