Renderer "middleware" for additional file modifications

[eikenb]

Add a way to make the renderer, the part that takes the evaluated template text and writes it to a file, extensible with post (and pre?) write effects. Like creating a backup, changing permissions or owners.

It came up in a recent issue hashicorp/consul-template#1497 that Nomad already uses the file permissions code in consul-template and there is a need for them to set the UID/GID of the files as well. In order for hashicat to replace consul-template it will need to support these sorts of features.

The code currently has a placeholder/reminder setup for this with the backup command, replace that with this.

[eikenb]

Couple more references I wanted to capture.

Nomad ticket about adding UID/GID support to templates.
hashicorp/nomad#5020

PR to add this feature to consul-template.
hashicorp/consul-template#1531

[eikenb]

Merged hashicorp/consul-template#1531... so going to need this before I can migrate those changes.