Clay Menu Plugins

[jonwinton]

Goal

We need the option to put plugins into the left drawer.

(https://github.com/clay/clay-kiln/blob/master/lib/nav/nav-menu.vue)

Proposed Solution/Implementation

Current implementation:

'use strict';

module.exports = () => {
  window.kiln.toolbarButtons['article-syndication'] = require('./button.vue');
  window.kiln.modals['article-syndication'] = require('./modal.vue');
};```

We need a spot for plugins to register themselves with the left drawer

## Documentation Questions
- How is it documented?
- Why do we put plugins here vs other spots?