Add builtin custom matcher function

Requested feature Add builtin custom matcher function

Why the feature is needed Currently, we should implement Matcher interface in our own code, and it can be useful to provide the ability to create custom matcher using a function in the lib.

Proposed solution We can create a constructor function, that accepts fmt.Stringer, a match function and returns a new matcher.

	mockMatcher.EXPECT().Matches(gomock.Custom(
		gomock.StringerFunc(func() string {
			return `expect string contains"test"`
		}),
		func(i interface{}) bool {
			v, ok := i.(string)
			if !ok {
				return false
			}
			return strings.Contains(v, "test")
		},
	)).Return(true)