Add the ability to prefix env var name with SQLPAGE_

Fixes #44

Author: lovasoa

configuration.md

@@ -23,6 +23,8 @@ All the parameters above can be set through environment variables.
 The name of the environment variable is the same as the name of the configuration variable,
 but in uppercase.
 
+The environment variable name can optionally be prefixed with `SQLPAGE_`.
+
 ### Example
 
 ```bash

src/app_config.rs

@@ -34,6 +34,7 @@ pub fn load() -> anyhow::Result<AppConfig> {
         .set_default("listen_on", "0.0.0.0:8080")?
         .add_source(config::File::with_name("sqlpage/sqlpage").required(false))
         .add_source(env_config())
+        .add_source(env_config().prefix("SQLPAGE_"))
         .build()?
         .try_deserialize::<AppConfig>()
         .with_context(|| "Unable to load configuration")?;