Multiple buttons

d.o issue: https://www.drupal.org/node/2273361

Note: This is just the first step in order to support different types of entities for selection. Read my last comment (short essay: https://www.drupal.org/node/2273361#comment-8999597) on the issue for more details.
Only generic entity embed button is enabled by default. You can enable per entity type buttons in the <a href="@settings_page">settings page</a>. When you enabled them go to <a href='@filterformats'>Text formats and editors</a> configuration page to add them to CKEditor's toolbar.