Make it possible to use env. variables in config.

If there is an env. variable FOO=bar, then we replace $FOO with bar in
the configuration.

Author: brocaar

src/config.rs

@@ -1,4 +1,4 @@
-use std::fs;
+use std::{env, fs};
 
 use anyhow::Result;
 use serde::Deserialize;
@@ -83,6 +83,11 @@ impl Configuration {
             });
         }
 
+        // Replace environment variables in config.
+        for (k, v) in env::vars() {
+            content = content.replace(&format!("${}", k), &v);
+        }
+
         let config: Configuration = match toml::from_str(&content) {
             Ok(v) => v,
             Err(err) => return Err(anyhow!("parse config file error: {}", err)),