Allow loading CRD data from a local file

[VSpike]

We've use this patch on a fork for about 18 months, and it's very useful for working with libraries where they don't make the CRDs available via a single, simple web endpoint.

We often have local Makefiles which pull down manifests and process them before passing them to k8s-gen to write out the libraries.

I've added the patch to a PR here #380 as it would be great to get it merged upstream if possible.