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