feat(oauth2): support passing extra query params (#358)

docs/schemes/oauth2.md

@@ -26,9 +26,17 @@ auth: {
 ```
 
 ```js
-this.$auth.loginWith('social')
+this.$auth.loginWith('social', options)
 ```
 
+`options` is an optional object with `params` property defining additional URL parameters to pass to authorization endpoint. For example:
+
+```js
+this.$auth.loginWith('social', { params: { lang: 'en' } })
+```
+
+will add extra query parameter `&lang=en` to a URL.
+
 ### `authorization_endpoint`
 
 **REQUIRED** - Endpoint to start login flow. Depends on oauth service.

lib/schemes/oauth2.js

@@ -63,7 +63,7 @@ export default class Oauth2Scheme {
     return this.$auth.reset()
   }
 
-  login () {
+  login ({ params } = {}) {
     const opts = {
       protocol: 'oauth2',
       response_type: this.options.response_type,
@@ -73,6 +73,7 @@ export default class Oauth2Scheme {
       // Note: The primary reason for using the state parameter is to mitigate CSRF attacks.
       // @see: https://auth0.com/docs/protocols/oauth2/oauth-state
       state: this.options.state || randomString(),
+      ...params,
     };
 
     if (this.options.audience) {