Adds `--chart` flag for overriding the selected release's chart ad-hoc-ly like `helmfile --chart $CHART template`.
This is handy when e.g. you want to have an ArgoCD application per each release in your helmfile.yaml, while also providing the ability to customize the release's chart without touching helmfile.yaml.
See https://github.com/roboll/helmfile/issues/1690#issuecomment-812321354 for more context.
Closes#1690
a161796dc4