[FEATURE] Use variables in file paths

[tezkerek]

Is your feature request related to a problem? Please describe.
I'd like to use variables in dotter's config files themselves. Something like:

[variables]
config_dir = "~/.config"

[shell.files]
"zsh/zprofile" = "~/.zprofile"
"zsh/zshrc" = "{{config_dir}}/zsh/zshrc"
"gitconfig" = "{{config_dir}}/git/config"

and then in ~/.zprofile too:

XDG_CONFIG_HOME="{{config_dir}}"

Describe the solution you'd like
Maybe parse the variables first, then render a handlebars template for each path. Or is that overkill?
Alternatively, maybe new values can be added to shellexpand, to be substituted like env vars ($config_dir/zsh/zshrc). Might need changes upstream. I think shellexpand::env_with_context could be used for this.

Describe alternatives you've considered
Run dotter with variables set on the shell for shellexpand to use:

env XDG_CONFIG_HOME="~/.config" dotter deploy

[SuperCuber]

Currently you could just use the variable itself in the path since it's expanded (well, after running dotter once and restarting the shell I guess)

Maybe parse the variables first, then render a handlebars template for each path. Or is that overkill?

That'd be the only way of doing it. I'll test if this brings a significant slowdown, if it doesn't then I don't see a reason against this.

[tezkerek]

I looked over shellexpand and I think that a custom context would work too, and might be more lightweight than a handlebars template. But having helpers in paths might be useful anyway.

[SuperCuber]

Yeah, if I'm adding access to variables might as well have full handlebars functionality, with handlebars syntax.

[murkylife]

This would be pretty helpful to me. I'd like to do something like:

includes = [".dotter/{{OS}}.toml"]
OR
includes = [".dotter/$OS.toml"]

This will allow me to dynamically get the correct OS files.