[ENHANCEMENT] LINE_IN_FILE as hook for templates

[tobiastom]

Is your feature request related to a problem? Please describe.
Sometimes one would like to write dynamic content to a file. This works with the render hook inside templates.
Unfortunately some files need to be merged from different templates. My concrete example is the domains.txt from dehydrated.

I'd like to create a template for each hostname I configure in Nginx. When I use RENDER to render this file with the HOSTNAME argument, the previous content is replaced.

Describe the solution you'd like
I'd like to use something like this inside a template:

LINE_IN_FILE ${HOSTNAME} /usr/local/etc/dehydrated/domains.txt

This would check if the line exists inside the file, and if it doesn't appends it.

Describe alternatives you've considered
I can run a custom command inside the jail, which works, but is not very convenient:

CMD grep -qxF ${HOSTNAME} /usr/local/etc/dehydrated/domains.txt || echo ${HOSTNAME} >> /usr/local/etc/dehydrated/domains.txt