Add support for Kustomize to fab generate. Implementation of #239. (Recovered)

[andrewDoing]

Note: I'm remaking this PR because while cleaning my branches I removed the changes from the branch used in the previous PR. I had to rewrite the documentation of the functions, but otherwise is the same as before.

This pull request adds a -k or --kustomize option to fab generate. When this option is passed, a kustomization.yaml will be generated for each environment with the Kubernetes YAML files added as resources. This allows executing kubectl apply -k on directories with the generated files.

This PR implements #239, and was inspired by #240

Refactored kustomize code into its own file
Added tests
Updated generate.go, adding kustomize to the command line options
Updated generate_test.go, adding the false parameter for kustomize (fixes the tests)
Added "kustomize" go the .gitignore so the tests can generate kustomize.yaml files without adding them to the working tree.

[andrebriggs]

@andrewDoing we need to add documentation for this. cc @evanlouie

[andrebriggs]

Mentioned above

[fire-ant]

Kustomize looks like a delightful addition to fabrikate and could kustomize would be a welcome addition for a few parties (from what I can see on issues). I'm only beginning to look at fabrikate/bedrock but I feel like I'd need kustomize support pretty early on to get the party started. Appreciative either way!