Made this as an issue in github, but realized it might be more appropriate here.
Note: I would be willing to implement this if others think it is valuable
In working with another generator, I needed a function that didn’t exist in the funcMap, and adding it required a PR to the generator. It occurred to me that it would be very easy to implement a plugin system to allow users to implement their own functions in the funcMap.
Here’s my suggestion - plugins are very simple executables that follow this pattern:
If a plugin executable is run with no arguments, it should output to stdout a newline delimited list of function names. Hugo would run all the plugins at startup and keep a reference of which plugin produced which function name, for when they’re called from a template.
When a template calls one of the plugin’s functions, the plugin executable is run with with the function name as an argument, and any further arguments are sent as a json array to the plugin’s stdin. The result of the program should be json formatted and written to stdout. If an error is encountered, the executable should exit with a non-zero exit code, and write any error text to stderr.
Bam, now any user can create a plugin in any language they want to extend the functionality of Hugo to support any function they want.
 How plugins are discovered by Hugo is left to the developer. I don’t really care if it’s a magic directory or specially named executables in the $PATH or whatever.
 We do this to avoid OS character limits on executable arguments.