Renderer "middleware" for additional file modifications

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 https://github.com/hashicorp/consul-template/issues/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.

Start by locating the renderer that evaluates template text and writes files, then inspect the existing backup-command placeholder and issue #1497 for context. The work is done when the renderer has a defined extension point for pre- and post-write effects, including backup handling and file permission or UID/GID changes.

Written by the indexing model from the issue text.