Update pattern for templated golden files

[gtrevg]

First, thanks for building this package!

I'm trying to leverage the goldie.AssertWithTemplate feature along with the pattern of using the -update flag when the file doesn't exist.

In regards to the template golden file, I would expect the process to be something like:

• Write unit test
• Run go test and see it fails with the use -update flag message
• Run go test -update and see the example.golden file gets created
• Replace Golden in the generated file with {{ .Type }}
• Run go test and see everything passes
• Write some table driven tests that iterate through many different types

At that point, everything seems good. Then we

• Create some more unit tests in that package
• Need to generate the initial golden files for those tests, so we run go test -update
• Run go test to validate
• Tests fail because the templated example.golden file is now overwritten with a single value and doesn't retain the initial {{ .Type }} that we put in there.

I can imagine that it would be relatively simple to go back and change the one overwritten .Type variable in this case. It is a bit more complex, though, if you had a bunch of variables throughout a large .golden file.

Do you have any thoughts on how to preserve the original template variables while doing an -update? One idea is that on -update, the template code could be smart enough to replace all scalar values in the data passed in with {{ .Dot.Path.To.Key }}. That way, when .Type writes itself out to the golden file, it'll end up writing itself out as {{ .Type }} once again, thus preserving the template variables while updating all the other content of the golden file.

Thanks!