Feature request - allow overriding of filename

[patmacpherson-sc]

Currently, the files are named after the executed test.

However, if you want to run multiple approvals in a single test (for example, with a table driven test) then the different approvals will overwrite each other.

It would be nice to have a function like-

func VerifyWithFilename(t Failable, reader io.Reader, filename string) 

which overrides the default naming behaviour.

[hjwk]

I think this can be easily added, I will work on a PR for it soon