xpip install -U 'xonsh[full]'

[jgart]

Hi Guixers,

When installing xonsh I get the following after starting:

```
You are currently using the readline backend. For interactive tab-completion, 
on-the-fly syntax highlighting, and more, install prompt_toolkit by running:

  xpip install -U 'xonsh[full]'

```

How should Guix handle optionally installing xonsh[full]?

all best,

jgart