Guix implementation for bullet-train

[Phil <phil@beadling.co.uk>]

Hi all,

I've implemented a basic Guix plugin for Oh-My-Zsh's Bullet Train theme.

I plan to add more functionality going forwards as I think of more use cases.

Bullet Train is a fancy console prompt that supports Python, Ruby, Git, AWS, etc
- by augmenting the command prompt with contextual about the current
console environment and directory location.

I based the Guix plugin on the Python virtualenv one - but thinking
about how to represent Guix environmental data, it's more complex than
virtualenvs as it's not just a binary virtualenv is active or not.

I've made some simple assertions below - if anyone has an opinion on
this let me know.

- Guix profiles can be stacked on top of each other, and the order is
  relevant.  Unlike virtualenvs, we can source many profiles and we need
  to be able to visually reference this.  I do this by listing profiles
  in the order they were sourced from left to right:
  .guix_profile ← my_profile ← some_other_profile
- Guix environments (excluding --pure) can also be added to this stack
  with environments running on top of profiles, or you can source a
  profile from inside an environment.  However when an environment is
  exited it and any profiles stacked on top of it are lost, and must be
  pruned from the visual representation:
  .guix_profile ← my_environment ← some_other_profile

  But on exit from the environment the visual representation will be
  pruned to this:
  .guix_profile


Thanks,
Phil.

My fork:
https://github.com/falloutphil/bullet-train.zsh

My changes:
https://github.com/caiogondim/bullet-train.zsh/compare/master...falloutphil:master