[wip] Add SubClass abstract class

[retrofox]

Some methods / endpoints have the same pattern. For instance site.category, site.tag, site.media, site.post have get, add, update and delete methods. So to trying to reduce and simplify the code I've created this SubClass class. I am all ears to change its name.

import `SubClass` from `./subclass`;

class SiteCategory extends SubClass {
  constructor( slug, siteId, wpcom ) {
    super( {
      type: 'sites',
      subtype: 'categories',
      idBySlug: true,
      id: slug,
      baseId: siteId,
      wpcom
    } );
  }
}

// then ...

wpcom
  .site( 'es.blog.wordpress.com' )
  .category()
  .add( { name: 'Orange', description: 'Orange is the new black' } )
  .then( res => {
    // response is here ...
  } (
  .catch( err );