Make the generated header comment customisable

[gittycat]

The current header (below) is hardcoded. This needs to be made customisable so that we can give more specific information on how the code is generated in each specific instance (could be through Makefile, through documented process, through go generate, ...).

header:

// This file was automatically generated by genny.
// Any changes will be lost if this file is regenerated.
// see https://github.com/cheekybits/genny

[matryer]

You are welcome to send a PR.

I would suggest that you can only add to the comment above, not replace it altogether.

[gittycat]

That's fine. In that case we can simply add a custom heading in our template.go file. No need to change genny's header.