Remove username as a required credential for Instagram

Default Instagram username to `self` unless ->setUsername() is specified

README.md

@@ -76,7 +76,6 @@ use SocialProof\SocialProof;
 
 return SocialProof::social()
   ->instagram()
-      ->setUsername('username')
       ->setToken('XXXXXXXXXXXXXXXXXXXXXXXX')
   ->get();
 ```

src/Providers/Instagram.php

@@ -27,13 +27,19 @@ class Instagram implements ProviderInterface
      */
     protected $endpoint = '/v1/users/';
 
+    /**
+     * Default username for this provider.
+     *
+     * @var string
+     */
+    protected $username = 'self';
+
     /**
      * Credentials required for this provider.
      *
      * @var array
      */
     protected $credentials = [
-        'username',
         'token'
     ];
 
@@ -66,6 +72,8 @@ public function get($credentials, $config)
         $config->api = $config->api ?? $this->api;
         $config->endpoint = $config->endpoint ?? $this->endpoint;
 
+        $credentials->username = $credentials->username ?? $this->username;
+
         $instagram = $this->client($config)->getAsync($credentials->username, [
             'query' => ['access_token' => $credentials->token]
         ])->then(