Quality of life changes (#968)

* + Added function to get a user's default avatar
+ Added HOST constant to Constants
+ Added assets endpoint
+ Added quality of life function to get a user's avatar or default avatar
+ Added quality of life function to get member's nickname or username

* * Fixed invocation of a getter.

* Fixed lint issue

* Made the API constant use the HOST constant for DRY-ness
Changed DOC comment to be more descriptive

* Update GuildMember.js

src/structures/GuildMember.js

@@ -182,6 +182,15 @@ class GuildMember {
     return this.user.id;
   }
 
+  /**
+   * The nickname of the member, or their username if they don't have one
+   * @type {string}
+   * @readonly
+   */
+  get displayName() {
+    return this.nickname || this.user.username;
+  }
+
   /**
    * The overall set of permissions for the guild member, taking only roles into account
    * @type {EvaluatedPermissions}