Feature request - allow overriding of filename

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.