[PATCH] gnu: Add direnv

[Christopher Baines <mail@cbaines.net>]

direnv is an environment switcher for the shell. direnv can be used with many
tools, including Guix environments.

I was lucky enough to meet the original author in London, who showed me direnv,
and helped me write a function for my .direnvrc file to make it easy to use
with Guix.

I have the following in my .direnvrc file (in my home directory).

    use_guix() {
      eval "$(guix environment "$@" --search-paths)"
    }

This means that I can have something like the following in my .envrc files

    use_guix --ad-hoc guile

You need to add a shell hook, and "allow" this file, but after that is done,
just changing in and out of the directory with the .envrc file in it will
adjust the environmental variables in your shell.