Add "--output-dir-template" flag to "helmfile fetch"

[elchenberg]

It would be great to have more control over the structure of the output directory of the helmfile fetch --output-dir string command with a --output-dir-template flag.

Currently the default structure seems to be {{ .OutputDir }}/{{ .Release.Name }}/{{ .Release.Chart }}/{{ .Release.Version }} but I would like to override it with only {{ .OutputDir }}, for example.

(Inspired by the "helmfile template" flag of the same name.)

[yxxhero]

@elchenberg PR is welcome.

[elchenberg]

Thanks, I'm on it. 🙂