Add custom matcher for verifying generator function replacement

This MR was part of !62 (closed)

This MR provides a custom matcher, SatisfyReplacement, that can be used to programmatically verify the replacement of the generator function. A basic implementation of the matcher is provided.

Here is an example:

generated := gitlabctl.ShellDeployment(gitlab)
templated := template.Query().DeploymentByComponent("gitlab-shell")
Expect(templated).NotTo(BeNil())
Expect(templated).To(SatisfyReplacement(generated))

/*  ignore specific fields, e.g. ObjectMeta.Name and ObjectMeta.Labels, etc. */

Expect(templated).To(
    SatisfyReplacement(generated,
        IgnoreFields(metav1.ObjectMeta{}, "Name", "Labels"),
        IgnoreFields(appsv1.DeploymentSpec{}, "Strategy")))

Related to #33 (closed)