⚡ improvement(index): support retunable missing handler (#256) by @ho…

…ud1ni

Missing() can produce some dynamic results, using it's own fallback algorythm and make something like 'header.nav.toHome' -> 'To Home' transformation e.g. It's would be interesting to make it async, but it seems that it's a little bit deeper change.

src/index.js

@@ -171,8 +171,11 @@ export default class VueI18n {
 
   _warnDefault (locale: Locale, key: Path, result: ?any, vm: ?any, values: any): ?string {
     if (!isNull(result)) { return result }
-    if (this._missing) {
-      this._missing.apply(null, [locale, key, vm, values])
+    if (this.missing) {
+      const missingResult = this.missing.call(null, locale, key, vm)
+      if (typeof missingResult === 'string') {
+        return missingResult
+      }
     } else {
       if (process.env.NODE_ENV !== 'production' && !this._silentTranslationWarn) {
         warn(