register_preprocessor: Register a resource preprocessor
In syberia/director: Load, track, and manage dependencies in a file structure.

Description Usage Arguments Examples

1	register_preprocessor(path, preprocessor, overwrite = FALSE)

`path`	character. The prefix to look for in the director.
`preprocessor`	function.
`overwrite`	logical. If `TRUE`, `register_preprocessor` will overwrite the route instead of erroring if the path already has a registered preprocessor. The default is `FALSE`.

## Not run: 
  d <- director("some/project")
  d$register_preprocessor('models', function() { print("I am a ", resource, ", a model!") })
  r <- d$resource("models/some_model.R")
  r$value() # Will print: I am a models/some_model, a model!

## End(Not run)