refactor: Rewrite cmd_duration, directory and env_var module to use module config (#460)

This PR is a batched rewrite of the following modules:
- cmd_duration
- directory
- env_var

src/configs/env_var.rs

@@ -0,0 +1,29 @@
+use crate::config::{ModuleConfig, RootModuleConfig, SegmentConfig};
+
+use ansi_term::{Color, Style};
+use starship_module_config_derive::ModuleConfig;
+
+#[derive(Clone, ModuleConfig)]
+pub struct EnvVarConfig<'a> {
+    pub symbol: Option<SegmentConfig<'a>>,
+    pub variable: Option<&'a str>,
+    pub default: Option<&'a str>,
+    pub prefix: &'a str,
+    pub suffix: &'a str,
+    pub style: Style,
+    pub disabled: bool,
+}
+
+impl<'a> RootModuleConfig<'a> for EnvVarConfig<'a> {
+    fn new() -> Self {
+        EnvVarConfig {
+            symbol: None,
+            variable: None,
+            default: None,
+            prefix: "",
+            suffix: "",
+            style: Color::Black.bold().dimmed(),
+            disabled: false,
+        }
+    }
+}