Support generation of Examples

[tmc]

[cweill]

@tmc: Can you please provide an example of what this would look like?

[Azero-NG]

I think the following is what i and he need

source file

func XXX() {
}

generated example function in test file

func ExampleXXX() {

    //Output:
}

[Azero-NG]

if gotests have this feature, we will generate example functions quickly and easily like generating test functions.

[cweill]

If anyone is willing to submit a PR, I'll review it.