Support shell "decorations"

[RadicalZephyr]

Basically, support some concept of config "mixin" or layering. This could be helpful configs for a specific project, or general use things for a particular language that for whatever reason you don't want to live in your main config.

Or perhaps represent cross-cutting concerns that you always want included in configs, no matter what shell is being used...

Better name is probably needed. Although "decorator crabs" are a thing for crabs that choose to bits of stuff on their shells to disguise themselves...

[RadicalZephyr]

I don't think there needs to be a separate concept for layering in another config. However, we could support two features.

• Individual repo/layer .hermitignore files, or alternatively an include list of which files/folders should be symlinked.
• Arbitrary rooting for that layer.

This means you can pick and choose on a per-shell basis what directories/files are included from a repository, and then "root" that directory into your home directory at any place. This means that there is some bookkeeping associated with each layer.

I think this metadata about how to include a repository should be done in such a way that it is commitable into your base repo. So, at a minimum a file containing a list of repository remotes to pull in, the root for that repository and a safe-list of files/directories.

I'm not really sure why, but having a list of directories to include from a layer feels better than including everything by default. Not sure why though.