feat: use runtimeConfig to set options (#250)

Nuxt allows you to use environment variables through useRuntimeConfig. This is preferable when you
want different settings in different environments, and want to use secrets (e.g. SMTP username and
password) without exposing them in nuxt.config.ts. This change adds `runtimeConfig.mail` options to
the options object.

fix #120

src/index.js

@@ -22,7 +22,14 @@ const moduleName = parsePackagejsonName(packageConfig.name).fullName;
 
 export default function (moduleOptions, nuxt) {
   nuxt = nuxt || this;
-  const options = { ...nuxt.options.mail, ...moduleOptions };
+
+  const runtimeConfig = useRuntimeConfig();
+
+  const options = {
+    ...runtimeConfig.mail,
+    ...nuxt.options.mail,
+    ...moduleOptions,
+  };
 
   if (!options.smtp) {
     throw new Error('SMTP config is missing.');