Support for `kind: List`

[m0un10]

Addresses https://github.com/bryanl/manifest-summary/issues/4

This change adds support for kind: List so that manifest-summary will work with tools in the Kubernetes ecosystem that use the List kind.

Some examples:

kompose convert --stdout --suppress-warnings  | manifest-summary
k get pods -oyaml | manifest-summary

I had to also introduce go.mod (and thus remove the vendor directory) in order to be able to use this with the newer versions of go.