Add configuration variables

[coastalwhite]

Add variables which can be inserted onto other configuration options

This would look like:

variables = {
    focus-color = "#FF0000",
}

# ... snip

switcher_color = "${focus-color}"

[danma3x]

I've looked into this, and my current suggestion would be rather hacky. Read the toml into a String, have a separate struct implementing Deserialize parse the variables into a HashMap, substitute the variables in-memory using regex, deserialize the resulting config, merge with default configuration. If that sounds reasonable enough, I'll start working on a PR for it.

[coastalwhite]

I think this is a good way to initially implement it. I would be very open to seeing a PR with this.