env: add a 'convert_case' field to ease dealing with kebab-case

This allows usage of `kebab-case` attribute in serde, mapping
unambiguously into a config value given a multiple character separator.

This also add the `convert-case` feature.

For example:

    let environment = Environment::default()
        .prefix("PREFIX")
        .translate_key(Case::Kebab)
        .separator("__");

1 files changed, 55 insertions, 0 deletions

diff --git a/tests/env.rs b/tests/env.rs
index 3a24bde..a144d08 100644
--- a/tests/env.rs
+++ b/tests/env.rs
@@ -464,6 +464,61 @@ fn test_parse_string_and_list() {
 }
 
 #[test]
+fn test_parse_nested_kebab() {
+    use config::Case;
+
+    #[derive(Deserialize, Debug)]
+    #[serde(rename_all = "kebab-case")]
+    struct TestConfig {
+        single: String,
+        plain: SimpleInner,
+        value_with_multipart_name: String,
+        inner_config: ComplexInner,
+    }
+
+    #[derive(Deserialize, Debug)]
+    #[serde(rename_all = "kebab-case")]
+    struct SimpleInner {
+        val: String,
+    }
+
+    #[derive(Deserialize, Debug)]
+    #[serde(rename_all = "kebab-case")]
+    struct ComplexInner {
+        another_multipart_name: String,
+    }
+
+    temp_env::with_vars(
+        vec![
+            ("PREFIX__SINGLE", Some("test")),
+            ("PREFIX__PLAIN__VAL", Some("simple")),
+            ("PREFIX__VALUE_WITH_MULTIPART_NAME", Some("value1")),
+            (
+                "PREFIX__INNER_CONFIG__ANOTHER_MULTIPART_NAME",
+                Some("value2"),
+            ),
+        ],
+        || {
+            let environment = Environment::default()
+                .prefix("PREFIX")
+                .convert_case(Case::Kebab)
+                .separator("__");
+
+            let config = Config::builder().add_source(environment).build().unwrap();
+
+            println!("{:#?}", config);
+
+            let config: TestConfig = config.try_deserialize().unwrap();
+
+            assert_eq!(config.single, "test");
+            assert_eq!(config.plain.val, "simple");
+            assert_eq!(config.value_with_multipart_name, "value1");
+            assert_eq!(config.inner_config.another_multipart_name, "value2");
+        },
+    )
+}
+
+#[test]
 fn test_parse_string() {
     // using a struct in an enum here to make serde use `deserialize_any`
     #[derive(Deserialize, Debug)]