feat: execute templates on content

[bketelsen]

Add the ability to execute a template on the entire dataset, on a single set of models in the dataset, or once per model in a single set of models.

[bketelsen]

imagine adding a -o flag for output filename that had parameterization in it based on the content model:

blox execute -t article.txt.tmpl -w articles -e -o '[id].go'

[bketelsen]

might consider renaming this to blox template because having blox exec or blox execute with roughly the same format, but instead of running a template, it executes a shell command, would be awesome.

[github-actions]

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